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A visible wavelengths spectrophotometer is constructed from a LED light source, CD/DVD "diffraction grating" and LDR/photodiode as a detector, with PIC microcontroller/stepper motor for control and output.

We have been growing vegetables hydroponically for a number of years. This obviously requires making and supplying nutrient solution to the plants. Since the initial formulation consists of weighing the appropriate amount of various chemicals and diluting with a set amount of water, it is relatively easy to obtain the correct initial concentrations. It is then generally recommended (1) that the nutrient solution is discarded and renewed on a regular basis (normally a time period of a few weeks to month). This is relatively wasteful and a better option would be to monitor the concentration of the various nutrients to determine uptake rate by the plants (and other losses) and then add specific chemicals as and when needed.

Measuring pH and electrical conductivity is straight forward, and relatively inexpensive instruments are available. However, pH and electrical conductivity (while important) are measurements of the "bulk" of the nutrient solution, and measuring specific nutrients such as nitrogen, potassium, phosphorus etc is crucial. Unfortunately, "wet chemical" methods for such nutrients (i.e. using beakers/burettes and titrimetry or gravimetry), which possibly could be used in the DIY setting, are not useful at the concentrations of interest. Generally, such compounds are measured using colorimetry or spectrophotometry (i.e, adding a specific reagent that makes a specific colour with the nutrient of interest, and the "deeper" the colour, the higher the concentration of the nutrient).

Commercially available colorimeters and spectrophotometers are expensive (particularly for spectrophotometers) and unless the hydroponics was of a "commerical scale" such equipment is not warranted for the small DIY/backyard setup (i.e. cheaper just to make the nutrient solution fresh every few weeks as recommneded in the literature). However, a colorimeter/spectrophotometer is just a light source with a method of discriminating "colours" which are passed through the test solution and measures the amount that exits (or was absorbed/blocked by the solution). The amount of particular "coloured" light that is absorbed/blocked by the solution allows calculation of the concentration of a particular nutrient.

Using currently available high current LED's (light source) coupled with light dependent resistors/diodes (detector) and various methods for diffracting or filtering light, enables the possibility of constructing a DIY colorimeter/spectrophotometer. This has indeed been done and reported by several authors (2), (3), (4). Using this information, a spectrophotometer was constructed and tested as detailed in the various sections below.

Spectrophotometer Components

A spectrophotometer is composed of a photometer and a spectrometer, acting in conjunction. A photometer is a device for measuring the intensity of electromagnetic radiation (i.e, visible light in this case). Whereas, a spectrometer is a device that can disperse electromagnetic radiation so that individual wavelengths can be used/measured as opposed to the total incident radiation (i.e, in this case individual "colours" from the "rainbow" of colours from normal "white light").

The "original" colorimetry uses the property of chemical solutions that the observed colour of the solution changes with concentration of the constituents. In the quantitative or analytical use of this property, the colour of the solution is manipulated usually due to the formation of a specific coloured compound obtained by adding a specific reagent selective for the compound of interest (or it can also be inherent in some cases). The intensity of the colour formed is then compared to know intensities of solutions due to know amounts of the substance of interest.

If this comparision is done by eye, the simple instrument used is called a colorimeter (or color comparator). An example is pH for backyard swimming pools. A reagent tablet is added to a small amount of pool water which then changes colour from orange/red through to purple (or similar) depending upon acidity/alkalinity (ie pH). This colour is compared to a colour chart with the match giving the pH of the solution (various litmus papers work in a similar manner, with pH causing a colour change in the paper that is compared to a standard colour chart to "read" the pH).

This manual colour comparision can be improved by using filters (made from different coloured glasses/plastics which selectively absorb/transmit particular wavelenghts of light) and a suitable detector (LDR or photodiode etc). Using a particular filter to selectively transmit a particular "colour" the detector can report the intensity of the colour, and hence the concentration of the compound of interest. This type of instrument is a filter photometer. Due to the relatively wide range of wavelengths transmitted by each filter, this type of instrument can be of limited sensitivity.

Improving on the filter photometer is replacing the filters with a device that can "split light" into the constituent wavelenghts, i.e., a prism or diffraction grating, and thus specific wavelenghts can be used with the electronic detector measuring the intensity. Hence, spectrometer (spliting light into individual wavelengths) plus photometer (measuring intensity of light) gives a spectrophotometer. By using specific wavelengths, particular chemical components within solutions can be individually measured (with appropriate reagents/conditions to produce specific reactions giving specific "colours") at very low concentrations. Therefore, in order to produce a spectrophotometer we need a method to split light into consituent colours which can individually be transmitted through the test solution, with a detector to measure the intensity of the incident/transmitted light.

Diffraction Grating

The "spectrometer" part of the spectrophotometer requires a prism or diffraction grating. Diffraction gratings are usually the choice (for various reasons, e.g resolution, can be optimised for a wavelength etc) and also for the DIY choice, are more readily available generally. Indeed, a relatively useful diffraction grating can be made of an old DVD or CD (5). Not repeating how a diffraction grating works (just refer to a Google search if need be), the important relationship that is needed is for a reflection grating (if a DVD/CD is being used as a surrogate), which is:

sin α + sin ß = Nmλ

N : Number of grooves/lines per mm
m : Order of diffraction (m = 0, ± 1, ± 2,...)
λ : Wavelength
α : Incident angle (angle between incident light and the normal to the grating)
ß : Diffraction angle (angle between diffracted light and the normal to the grating)

Using this formula, the following table gives the calculated expected diffraction angle for various wavelengths versus incident light angle for a diffraction grating based upon a CD (i.e. groove/line spacing of 1.6um). This gives the expected geometery of the monochromator portion of the spectrophotometer.

The Testing/Experimental Results section gives details about actually using a DVD/CD as a diffraction grating. A LED light source and a CD are used to demonstrate resultant diffracted spectra ("rainbows") with observed angles compared to calculated angles that should be obtained.

Table 1: Calculated Diffraction Angle (beta) versus Incident Angle and Wavelength
 Diffraction Order
wavelength (nm) -1 0 1 2
Incident Angle (alpha) 10o
Incident Angle (alpha) 20o
Incident Angle (alpha) 30o
Incident Angle (alpha) 40o
Grooves per mm = 625 (CD groove spacing 1.6 µm)

Monochromator (Stepper Motor/Diffraction Grating)

The rotation of the DVD/CD diffraction grating needs to be controllable in small increments, remain stable when not changing position and be reproducible. Such functionality is most readily provided by a stepper motor. A stepper motor driver was constructed from a SN754410 IC and a PIC micrcontroller. The stepper motor is only required to rotate the diffraction grating, which is obviously light in weight, so the step size of the motor rather than the torque and or rotational speed is the specification of importance in this case.

A scrounged stepper motor from a disused printer, labelled as manufactured by Shinano Kenshi, model STH-39C013, 0.9o step and 21ohm was available. The step size of 0.9o is for full-step and or wave drive mode, whereas, 0.45o can be achieved using a "half step drive" (with the loss of some torque, which is not a concern in this situation - see here for more details about types of stepper drive).

With a set step size of 0.45o and using the diffraction grating equation given above (with groove spacing of 0.74µm for a DVD), taking the visual light spectrum as ranging from 750nm (red) to 400nm (violet) geometry can be used to give an indication of the range of wavelength passed by specific exit slit widths at various distances from the diffraction grating.

A exit slit of 0.5mm width at a distance of 60mm from the diffraction grating or a exit slit of 1.0mm width at a distance of 125mm from the diffraction grating would provide approximately 10nm bandpass. A 10nm "bandpass" is likely more than sufficient resolution to enable quantitative colorimetric analysis (albeit with limitations). The wider exit slit width would enable more light intensity to be directed through the test solution, and perhaps help with sensitivity. Whereas, the smaller exit slit would enable the spectrophotometer to be of small physical size/more compact. However, a wider entrance slit results in a larger image at the exit slit position, such that the image for the wavelengths adjacent to the target wavelength enters the exit slit and reduces the resolution.

Therefore, the smaller the step size of the stepper motor the finer can be the discrimination of adjacent "monochromatic images" of the entrance slit, using a suitably small exit slit. This lead to the eventual replacement of the initial stepper motor driver constructed from a SN754410 IC and the use of the A4988 Stepper Driver which enables microstepping down to 1/16th (notionally 0.056o with the Shinano Kenshi, model STH-39C013 stepper motor).

Using a light source to shine directly onto the DVD/CD diffraction grating via the entrance slit does not produce a linear dispersion of wavelengths. While such a "minimalist" approach will perhaps produce sufficient utility for DIY colorimetric analysis, additional optical elements are required to produce an actual "monochromator".

A Czerny-Turner mounting arrangement is used to explore an actual DIY monochromator more in line with the definition. The Czerny-Turner mount is relatively simple and only requires a pair of concave mirrors, in addition to the diffraction grating.

The Testing/Experimental Results section gives details about actually using a DVD as a diffraction grating mounted on the stepper motor and the results obtained from solutions containing dye compounds with known visible light absorption spectra.

This project is still under active development.

Details about LED light sources, stepper motor control and interface of sensors to PIC microcontrollers (i.e components currently being used in this project) can be found in the various pages in the Electronics Projects and Learning Electronics Sections (use menu at top-left hand side of this page).

Note: Image loading can be slow depending on server load.

  • RGB Photometer SchematicRGB Photometer Schematic

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    RGB Photometer Schematic

This project is still under active development.

Details about LED light sources, stepper motor control and interface of sensors to PIC microcontrollers (i.e components currently being used in this project) can be found in the various pages in the Electronics Projects and Learning Electronics Sections (use menu at top-left hand side of this page).

This project did not require a PCB.

The construction was done using prototyping board. See the photographs and schematic diagram sections.

Qty Schematic Part-Reference Value Notes
1R1, R210K1/4W, 10% 
1R35601/4W, 10% 
1R4 - R14101W, 10% 
1D1Red LED 
Integrated Circuits
1U1LA6358NDual Op-Amp 
1 fan12V fan
2 MetersMini DC 0V To 99.9V LED Digital Panel Voltmeter  

Some initial testing/proof-of-concept was performed in order to determine if potential individual DIY components of a spectrophotometer could indeed perform to a sufficient standard to enable a functional outcome when operated in conjunction. These individual functional components where grouped/tested as follows (and are discussed below in the corresponding sub-sections):

  • CD/DVD as a diffraction grating
    • DVD will give wider dispersion .: better resolution
  • Stepper Motor - to control diffraction grating angle
    • Small step size/control required to produce good resolution
  • LDR as detector
    • In conjunction with light source type/strength, entrance/exit slit width, gives sensitivity
  • RGB LED/photoresistor as a "filter photometer"

Diffraction Grating (CD/DVD)

A central component of a spectrophotometer is the diffraction grating used to produce the monochromatic light from the incident beam. As noted (3), (4), (5) a CD or DVD can be used as a diffraction grating. A CD has groove spacing of 1.6 µm whereas a DVD has groove spacing of 0.74 µm.

Table 1 in the Project Background section reports the expected diffraction angle for various wavelengths of light versus incident light angle to the normal of the diffraction grating (i.e. CD). A quick prototype/test of concept was developed to check if "monochromatic" light could indeed be generated that would be suitable as the basis of a spectrophotometer. This involved a 350mA white LED as light source (with a wall-wart power supply to a LM317 in constant current mode), a sliver of CD as the grating, and a piece of white paper as the detector screen (see Album 1 in the Photographs Section).

The incident white light (passing through a slit formed from old razor blades) was indeed split into the expected "rainbow" with the various colours at the expected angles. Therefore, by selectively rotating the grating (e.g. with a stepper motor or similar) a desired wavelength of light can be directed into a sample solution. Visually, the spectrum obtained was "bright" to the unaided eye (the photographs were taken with a 1/4 shutter speed, F=3.5) giving some confidence that the intensity will be sufficient for later transmission/adsorption/detection through a sample solution.

As expected from the literature/how diffraction gratings work, the 1st order spectrum is much "brighter" than the 2nd order diffraction spectrum. Although the 2nd order spectrum has a wider dispersion angle (which would give better resolution). However, the DVD (with closer groove spacing) compared to the CD gives similar dispersion with the 1st order as does the 2nd order with a CD. Therefore, the DVD with a 1st order diffraction spectrum would be the choice.

Light Source

The 350mA white LED was replaced with a WS2812 RGB LED as the light source. The WS2812 contains three seperate LEDs each producing a narrow band of wavelengths (Red 620-630nm, Green 515-530nm and Blue 465-475nm) - see photograph 7 in Album 1 in the Photographs Section. The idea being that the LED itself already produces "monochromatic" light (of 10nm bandwidth in three regions of the visible spectrum) and the diffraction grating (and associated stepper motor etc) could then be eliminated.

In effect, the WS2812 would provide the equivalent function of the "filters" in a filter photometer. The potential advantage being much simpler construction/fewer components/lower cost/easier setup. However, disadvantages and possible limitations that may prove the idea infeasible include:

  • only the three wavelengths are available
    • depending upon the colorimetric reactions/analytes of interest this may or may not be significant
  • the WS2812 can only provide a maximum of 20mA to an individual LED
    • this may not be of sufficient intensity to enable sufficient sensitivity
  • the individual LED's in the WS2812 component are physically offset (although much closer than "normal" RGB LED's)
    • this means the light paths may need to be aligned, complicating construction somewhat
    • if alignment for WS2812 is needed, using a "normal" RGB LED may be advantageous, as higher currents can be used (increasing light intensity, improving sensititivity)

Testing the WS2812 with a CD as a diffraction grating, produced the expected diffraction angle for the various wavelengths as per equation 1 in Project Background Section. This was done to test the CD "diffration grating" with known wavelengths, rather than just the white light source. The following video was produced from still images of operating the WS2812. Note, the WS2812 needed to be located a distance from the slit in order to "align" the light beams from the individual LEDs within the WS2812 (i.e. the WS2812 was sufficiently far away from the slit that it formed a diffuse source).

While a RGB LED shows some promise as a light source in a pseudo "filter" photometer, since the LED's within a WS2812 need to be "aligned" to the entrance slit, this means no particular advantage over a "normal" RGB LED. In fact, a normal RGB LED can be operated at higher current (possibly overcoming the limitation of needing to be situated at a distance from the slit to produce a "diffuse" source so all three individual LEDs within the RGB LED are "aligned") and the necessary control circuitry and software is much simpler.

Stepper Motor/Monochromator

Information in the Background Section explains the requirements and operation of a stepper motor to alter the incident light angle to the DVD "diffraction" grating in order to produce and direct monochromatic light through a test solution.

A rough working prototype was constructed (see Photographs Section Album 3) to initially test the feasibility of the proposed arrangement/components for a DIY spectrophotometer. Previous testing with a WS2812 RGD LED showed, as expected, that a DVD/CD does indeed diffract light and from the known relationship of wavelength and incident light angle, this diffraction angle can be used to select/determine/calibrate particular wavelengths of light to be used. However, in order for this to be useful in the DIY spectrophotometer, reproducible and fine adjustment to the angle of DVD/CD diffraction grating will be required, and additionally, the monochromatic light produced will need to be of sufficient intensity for the photoresistor to produce a reasonable response. A photoresistor was selected as a potential detector (rather than phototransistor or photodiode which would be more sensitive) because this component was available at the time.

The prototype consisted of:

  • A scrounged stepper motor from a disused printer, labelled as manufactured by Shinano Kenshi, model STH-39C013, 0.9o step and 21ohm, with a SN754410 stepper motor driver in "half step drive" mode;
  • DVD as a diffraction grating (groove spacing of 0.74µm) with an exit slit of 1.0mm width at a distance of 125mm from the diffraction grating (calculated to give approximately 10nm bandpass with above stepper driver step size);
  • GL5516 photoresistor (light resistance 10Lux 5-10Kohm, dark resistance 0.5Mohm) with a 220Kohm resistor to form a voltage divider with 2.5V voltage reference to a PIC 18F248 on board ADC via LM358 opamp as buffer (see Schematics Section);
  • White LED operated at 350mA as light source.
  • Firmware on PIC18F248 to operate stepper motor, ADC for reading the photoresistor voltage, and RS232 to PC for data transfer/collection

RGB LED Photometer

Previous Sections discussed the use of a WS2812 RGB LED instead of a white LED in conjunction with a diffraction grating. This would produce the equivalent of a "filter" photometer (i.e. a limited bandwidth within a limited portion of the spectrum rather than continuously variable selection of wavelength). Further, instead of using the WS2812 (which has a rather complicated communications protocol) subsitute a "normal" 4-pin RGB LED, which also enables the possibility of having greater light intensity than the WS2812 (which is limited to a maximum of 20mA). The advantage of such an arrangement is simplified construction with no moving parts (i.e. likely more robust). The disadvantage would be the limited wavelengths available. A rough prototype was constructed (see Photographs Section) and some initial testing performed using coloured food dyes.

A common-cathode RGB LED was used with the following specifications:

RGB LED Specifications
LED Colour Parameter Min Typical Max Unit
RedForward Voltage 1.882.2V
Luminous Intensity10001200 mcd
Peak Emission Wavelength 620630nm
GreenForward Voltage 3.03.2V
Luminous Intensity8001000 mcd
Peak Emission Wavelength515517.5 nm
BlueForward Voltage 3.03.2V
Luminous Intensity600800 mcd
Peak Emission Wavelength460462.5 nm
Peak Current 100 uA, specifications Itest = 20 mA

A LM317T was used in "constant current mode" (see Schematics Section) as a power supply, with a GL5516 CdS photoresistor as detector. The GL5516 has a light resistance (10Lux) of 5-10 Kohm, dark resistance of 0.5Mohm with a response time of ~30ms. The approximate spectral response curve is given in the following Figure 1.

As test solutions, commerical food dye (Queen Brand) was used, with the following consituents as stated on the product labels.

Food Colour Dye Constituents
Food Colour INS Food Code Constituent Details
Queen - RedColor 124Ponceau 4R, Cochineal Red A
Acid 330Citric Acid
Preservative 202, 211Potassium sorbate, sodium benzoate
Queen - GreenColor 133FD&C Blue Dye No1, Triphenylmethane dye
Acid 330Citric Acid
Preservative 211Sodium benzoate
Queen - BlueColor 102, 133Tartrazine (102), FD&C Blue Dye No1 (133)
Acid 330Citric Acid
Preservative 211Sodium benzoate

The following Figure 1 summarises the visible wavelength absorbance spectra for the dye compounds being tested, overlaid with the RGB LED emission wavelengths, together with the spectral response curve of the GL5516 photoresistor.

  • Figure 1: Visible Wavelength Spectral Data for RGB LED, Phototransistor and Dye Test Solutions

    Visible Wavelength Spectral DataVisible Wavelength Spectral Data

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    Figure 1: Visible Wavelength Spectral Data for RGB LED, Phototransistor and Dye Test Solutions

An initial proof-of-concept was performed using a cardboard enclosure to mount the photoresistor and the RGB LED (see Album 1, photograph 1 and 2 in the Photographs Section) as a "reading cell". The reading cell provides a means of focussing light from the RGB LED through a test solution (contained in a suitable container transparent to visible wavelengths) onto the photoresistor. The RGB LED was contained within the body of a disused pen to enable ease of direction/focus onto the photoresistor. A DMM was used to read the resistance of the RGB LED versus concentration of various dye solutions.

This initial testing provided the expected increasing resistance with increasing dye concentration (i.e. less light transmitted through the solution onto the photoresistor). The observed resistance versus dye concentration appeared to provide a potentially useful measurement range. The resistance measurement was relatively sensitive to the location of the glass vial containing the test solution (the round vial reflecting/refracting the light on different paths depending upon exact location in relation to the RGB LED and photoresistor).

This initial positive data lead to the manufacture of a more robust reading cell made from perspex scraps (see Album 1, photograph 3 and 4 in the Photographs Section). This perspex reading cell rigidly located the glass vial in relation to the RGB LED and photoresistor. Using various test solutions made from food colouring dyes, the prototype was tested to assess if the expected absorbances versus wavelength were observed and the likely sensitivity of the arrangement. Graphs 1 to 4 below present the results.

Before testing the various dye solutions, photoresistor resistance versus applied LED current was examined (graph 1). This shows increasing LED current (i.e. more "light") corresponds to a decrease in resistance of the photoresistor. Also, the decrease in resistance versus applied current is not linear (greater change in observed resistance at lower currents). This is as per the datasheet for the GL5516 CdS photoresistor, which reports resistance varies logarithmically versus applied light flux. This has implications that a linear relationship between solution concentration and observed photoresistor resistance may not result (i.e. the Beer-Lambert Law, increasing concentration and or path length of a solution will produce increasing absorbance in a linear relationship). A linear relationship is desirable for ease of calculation etc.

Interpreting the observed differences in Graph 1 between the individual red/green/blue LED's within the RGB LED is difficult. This is due to each of the individual LED's having different luminosities at the same applied current, the individual LED's within the RGB LED die are not exactly coincident (i.e. slightly different light paths in relation to the photoresistor), and the photoresistor responds different depending upon incident wavelength.

Test dye solutions were prepared by adding approximately 0.5ml of undiluted dye to 50ml of water. This amount of dye produced visually by eye an opaque solution (i.e. expected maximum absorbance/minimum transmitance). From this maximum concentration solution a series of other solutions were produced by serial dilution with water (see Album 1, photograph 8 and 9 in the Photographs Section).

Graph 2 shows the observed resistance of the photoresistor versus red dye concentration at various applied LED currents for each of red/green/blue LED's within the RGB LED. The red dye (containing cochineal, expected to absorb strongly around 500nm, i.e. corresponding to the emission of both the blue and green LED within the RGB LED) gives the expected absorption of both the blue and green LEDs, while transmiting the emission from the red LED. The absorbance of the blue LED is very sensitive (strongly non-linear), whereas, the green LED absorption is approximately linear (with the range of conditions tested).

Graph 3 shows the observed resistance of the photoresistor versus blue dye concentration at various applied LED currents for each of red/green/blue LED's within the RGB LED. The blue dye has a strong absorbance peak at approximately 630nm, corresponding closely with the emission peak of the red LED. The blue dye absorbance spectrum shows minimal absorbance at wavelengths corresponding to the blue and green LED's within the RGB LED. Graph 3 confirms this expected behaviour (and demonstrates good sensitivity). Due to the sensitivity observed, a further more dilute range of blue dye solutions were prepared. Graph 4 shows that results of measuring photoresistor resistance of these solutions against the red RGB LED at 20mA.

The observed results for the various dye solutions of measuring photoresistor resistance of these solutions against applied light from individual LED's within a RGB LED, shows that a "filter photometer" analogue using a RGB LED as light source with photoresistor as detector shows promise.

  • Graph 1: LED Current (mA) versus photoresistor resistance (Kohm)

    Graph 1Graph 1

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    Graph 1: LED Current (mA) versus photoresistor resistance (Kohm)

  • Graph 2: Photoresistor resistance (Kohm) versus Red Dye Concentration

    Graph 2Graph 2

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    Graph 2: Photoresistor resistance (Kohm) versus Red Dye Concentration

  • Graph 3: Photoresistor resistance (Kohm) versus Blue Dye Concentration

    Graph 3Graph 3

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    Graph 3: Photoresistor resistance (Kohm) versus Blue Dye Concentration

  • Graph 4: Photoresistor resistance (Kohm) versus Diluted Blue Dye Concentration

    Graph 4Graph 4

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    Graph 4: Photoresistor resistance (Kohm) versus Diluted Blue Dye Concentration

Czerny-Turner Spectrophotometer

Prior initial prototype work (reported above) showed that using a 1W white LED as a light source, with a DVD as a "diffraction grating" (mounted on a stepper motor) and a phototransistor as detector it is possible to obtain 'monochromatic' light through the visible spectrum, that can be directed through a test solution. However, this "minimalist" approach ignores that a parallel polychromatic light beam should be used for illuminating the diffraction grating, in order for the dispersion into monochromatic light to follow the grating equation. Also, since the incident light beam covers the surface of the diffraction grating, the dispersed light needs to be refocused onto the detector. Incorporating "collimation" and "refocusing" is relatively easily done (only requires two concave mirrors), and while complicating construction somewhat, will increase the likelihood of the DIY spectrophotometer having practical utility.

The Czerny-Turner arrangement (6) is used as this only requires a couple of easily sourced concave mirrors. The Wikipedia page (7) explains the elements of a Czerny-Turner monochromator (light source -> entrance slit -> collimating mirror -> diffraction grating -> focussing mirror -> exit slit -> detector). Therefore, the previous DIY spectrophotometer was augmented with two 15cm focal length concave mirrors.

Focal Length of Mirrors

The focal length of the mirrors was checked using both the "distant object method" and the "parallax method" (see Album 2, photograph 1 through 4 in the Photographs Section). The distant object method involved using the light from an illuminated slit (~1mm) directed onto the concave mirror with the reflected image viewed next to the slit (see Album 2, photograph 1). The mirror then being moved back and forth until the image of the slit is as sharp as possible (compare Album 2, photograph 2 and 3). The distance from the slit to mirror when the image is as sharp as possible is the focal length. The parallax method (8) uses two pins which are moved along the focal axis until the real inverted image of the 'object' pin is matched to the 'search' pin and no parallax can been seen between it and the real image. The distance of the 'object' pin and the 'seach' pin to the mirror are then used to calculate the focal length. For the purchased mirrors, both methods resulted in a focal length of 15cm (but since the mirrors were from a "cheap" supplier, best to first check rather than incorporate straight into prototype).

Grating Equation/Checking Dispersion

The grating equation (9) defines the relationship between wavelength and the incident and diffraction angle. This enables calculating from the known incident angle of the polychromatic light what is the expected diffraction angle of the monochromatic light of various wavelenghts. This was tested with a prototype to not only check if the desired dispersed spectrum could be achieved, but that the spectrum was at the expected angles (and thus the geometry of the components of the Czerny-Turner monochromator could be later designed). Using the grating equation and the law of reflection, the following diagram was produced using a 15cm focal length for the collimating mirror and printed to 1:1 scale.

  • Diagram 1Diagram 1

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    Diagram 1: Grating Equation Calculation of DVD Dispersion

The various components were physically mounted on this template (see Album 2, photograph 5) and the 1st order spectrum were observed at the expected angles (see Album 2, photograph 6 and 7). The white (polychromatic) light from the LED was collimated (parallel light beam) by the concave mirror and directed on the diffraction grating (cut from a DVD). The diffraction grating is assumed to be formed from a series of parallel straight grooves/lines. The DVD does provide the necessary parallel grooves, but obviously these are not 'straight' but curved. This can been seen in the 'curved' appearance of the resultant spectrum (Album 2 photograph 7). Note that the 'outside' edge portion of the DVD has been used as much as possible, as the DVD tracks in this region approximate 'straight' lines closer.

Alignment of optical components

A prototype Czerny-Turner spectrophotometer was constructed (Album 2 photograph 8) using a symmetrical arrangement based on 15cm focal length concave mirrors with a "DVD diffraction grating", using a 1W white LED as light source with a TCS3471 Color Light to Digital Converter as the detector.

In order to align the various optical components, an "optical rail" arrangement was constructed using a tongue-and-groove joint cut into a suitable piece of wood. This enabled "precise" placement of components both in a linear and parallel fashion, while still allowing flexibility in alterating the distances between components etc.

A Red Laser Pointer was mounted in the light source compartment which provided a monochromatic beam of 'known wavelength' (650nm) with which to align the mirrors, diffration grating and detector (Album 2 photograph 9 and 10). Photograph 9 in Album 2 also shows the 1st order diffraction of the red laser pointer light (the red dot on the 'backing screen'). The incident angle to the diffraction grating is 30o when the diffraction grating is perpendicular to the light path from the entrance slit to the collimator mirror. Therefore, with a wavelength of 650nm for the Red Laser Pointer and a groove spacing of 0.74um (DVD diffraction grating), the calculated 1st order diffraction angle is 22.2o. This was indeed the angle at which the the red dot on the 'backing screen' was observed.

This project is still in the prototyping, proof-of-concept phase. So construction has only been performed on breadboard to date.

Note: Image loading can be slow depending on server load.

Album 1: RGB Photometer

Album 2: Czerny-Turner Spectrophotometer

ref001: K. Roberto., How To Hydroponics, 4th Ed., www.futuregarden.com

ref002: http://www.instructables.com/id/A-simple-DIY-spectrophotometer/#step1

ref003: http://www.rsc.org/Education/EiC/issues/2007Sept/BuildYourOwn Spectrophotometer.asp

ref004: http://nznano.blogspot.com.au/2011/12/homemade-spectrometerspectrophotometer.html

ref005: http://astro.u-strasbg.fr/~koppen/spectro/experimtse.html

ref006: http://www.shimadzu.com/an/uv/support/fundamentals/ monochromators.html

ref007: https://en.wikipedia.org/wiki/Monochromator

ref008: http://physicsmax.com/to-measure-the-focal-length-of-a-concave-mirror-6838

ref009: http://www.horiba.com/us/en/scientific/products/optics-tutorial/diffraction-gratings/


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