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A simple webcam spectrograph

Ralph D. Lorenza)

Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland 20723

(Received 27 January 2012; accepted 8 December 2013)

A spectrometer is constructed with an optical fiber, a webcam, and an inexpensive diffraction

grating. Assembly takes a matter of minutes, and the instrument is able to produce quantitative

spectra of incandescent and fluorescent sources, lasers, and light-emitting diodes. Examples of data

analyses, carried out with free software, are discussed. VC 2014 American Association of Physics Teachers.

[http://dx.doi.org/10.1119/1.4853835]

I. INTRODUCTION

Spectroscopy is a powerful tool in astronomy, remotesensing, analytic chemistry, and many other fields. While thedispersion of light is readily demonstrated in the classroomthere is a “quantum leap” in expense between simple spec-troscopes, through which spectra can be observed by eye,and benchtop spectrometers that provide data for quantitativeanalysis. This paper attempts to bridge that gap by showinghow an instrument can be quickly assembled from a commonwebcam, and how quantitative data can be extracted foranalysis.

II. EQUIPMENT

The setup used is very simple and is shown schematicallyin Fig. 1. A fiber carries light into a dark box containing acamera with a grating. The grating used was a 1000 groo-ve/mm “Card-Mounted Diffraction Viewer” from EdmundScientific with a unit cost of less than $1. The plastic gratingfilm was cut from the card mounting and glued onto the focusadjustment ring of the webcam. The grating was located about3 mm in front of the lens, as shown in Fig. 2. For the conven-ience of the data analysis, it is preferred that the grating bemounted such that the dispersion direction is in the horizontaldirection of the image. The direction and the amount of dis-persion, which is a function of the groove spacing and the dis-tance between the grating and the lens, are readily visualizedby pointing the camera at a desk lamp or similar light sourceand observing the regular and the dispersed images.

Digital cameras of various types have proliferated dramat-ically in the last decade and can be used for various physicsexperiments,1 including the observation of diffractionphenomena.2 The use of a cellphone camera to perform

spectroscopy has been recently described.3 In the presentexperiment, we used a Logitech Quickcam with a 352� 280CMOS detector. This is a rather old system with a small de-tector but is adequate for our experiments. The camera wasremoved from its case for convenient mounting in the alumi-num die-cast box used for our setup, although this step is notalways necessary.

At the expense of modest sensitivity and spectral resolu-tion, no collimating and focusing optics are used beyond apinhole aperture to admit the light as a “point source.”Rather than a simple hole in the box, an optical fiber (e.g.,Jameco Electronics part #171272,� $6) was used to directthe light into the box. The fiber was inserted a couple of mminto a hole in the box and glued in place.

The webcam with grating was placed in the box and ori-ented so that it could view the fiber, as shown in Fig. 3.Important features of the box are stiffness and ease of access.It is important that the interior of the box is black to suppressreflections. The free end of the fiber was directed towards alight source so that the box end appeared as a bright dot inthe webcam image. The position of the webcam was adjustedso that the direct image of the fiber appeared close to theedge of the webcam sensor and the dispersed image near thecenter of the sensor. The webcam was secured in place withhot glue. The exact position depends on the sensitivity/reso-lution tradeoff desired and the size of the camera; placingthe camera closer to the fiber results in a brighter image butreduces the spectral resolution. In our setup, the camera waslocated about 8 cm from the fiber. We placed a slit betweenthe fiber and the webcam, as shown in Fig. 3, but theimprovement in spectral resolution was not worth the drop inlight intensity, and the slit was removed. For our measure-ments, the free end of the fiber was simply pointed at thelight sources of interest.

169 Am. J. Phys. 82 (2), February 2014 http://aapt.org/ajp VC 2014 American Association of Physics Teachers 169

It would be straightforward to add additional opticalfibers, preferably vertically above the first fiber, so that thedispersion is orthogonal to the fiber separation. Such anarrangement would make it easy to have a reference wave-length with which to compare a source under study or toexamine a white light source against the same source viewedthrough absorbing media, such as optical filters.

Portable spectrometers, such as those based on cell-phones,3 can be aimed at sources of interest, as could thepresent arrangement if the fiber were omitted. The fiber how-ever mechanically decouples the light source from the“optical bench” and thus is more flexible, particularly tostudy light sources under conditions that are not conduciveto proximate mounting of optoelectronic hardware. Anexample might be the study of the wavelength shift of light-emitting diodes when immersed in liquid nitrogen,4 whichmotivated the construction of our setup.

III. ANALYSIS SOFTWARE AND WAVELENGTH

MEASUREMENT

It is assumed that some kind of image acquisition utility isprovided with the webcam used. Plug-ins for other softwarepackages, such as National Instruments Labview or AdobePhotoshop, may also be available. It should be recognized

that any software can introduce transformations to the imagedata such as adjustments of contrast and corrections to non-uniformities in the spectral sensitivity of the three (RGB)types of pixels in the sensor (“white balance” corrections).Exposure times may also not be constant.

Our setup is sensitive enough to observe bright sourcessuch as LEDs, driven with 20 mA, or room lights. A candleflame, however, is barely detectable. If the exposure timecannot be controlled, image-stacking software, such asRegistax,5 can be used to combine many hundreds or thou-sands of short-exposure images to improve the signal-to-noise ratio although flat-fielding and dark subtraction steps,

Fig. 3. The setup of our webcam spectrograph. Although the slit improved

the spectral resolution, it resulted in a drop in light intensity and it was sub-

sequently removed. Simple hot glue was adequate to hold all components in

position. Aluminum tape was used to secure the webcam USB cable.

Fig. 2. A plastic transmission grating is held over the focus adjustment ring

of a webcam for trial measurements. In a more permanent setup, the card-

board can be removed.

Fig. 1. Schematic of the webcam spectrograph. The webcam images the end

of an optical fiber through a transmission diffraction grating.

Fig. 4. A montage of images of various light sources studied with the spec-

trometer. The direct (zeroth order) image of the fiber is visible on the right-

hand side in each image and appears to be much larger than the fiber dimen-

sions due to the strong intensity of the direct light. In the color rendering of

each image, the correlation between wavelength and image position in the

first-order spectrum is obvious.

170 Am. J. Phys., Vol. 82, No. 2, February 2014 Apparatus and Demonstration Notes 170

familiar to astronomers but perhaps less to others, arerequired for best results.

Figure 4 shows a montage of various light sources studiedwith our spectrometer. Each image is readily interpretablesince blue light appears as blue, red as red, and so on. Thewavelength of the light at each pixel is determined by theangle of diffraction. The color with which the light appearson the image is simply a function of the sensitivity of theRGB pixels at that location. The continuous spectrum of theincandescent light source is evident, as is the “notched”appearance of the white LED spectrum and the narrow-bandemission from the white fluorescent light source. LEDs areconvenient wavelength references. We note that the colorLED spectra have a horizontal smear; the spectra are a fewtens of nm wide. The red and green laser pointers have circu-lar first-order images that are crisp echoes of the zeroth-order fiber image, indicating that these laser pointers arehighly monochromatic light sources.

To process the images, the free analysis tool ImageJ,developed by the National Institutes of Health,6 was used.This tool allows a profile of the brightness along a line to beextracted and plotted or exported as a text file. The tempta-tion to simply plot the line of the image with the spectrum,not including the zero-order direct image of the source,should be avoided. The zeroth-order image should beincluded, because it defines the reference position fromwhich to measure the number of pixels to determine thewavelengths. If the setup is rigidly arranged and intended tobe permanent, the wavelength calibration should not changesignificantly and the raw pixel position from the image edgecan be an adequate reference.

The wavelength calibration of the spectrometer at the100 nm level can be carried out using the original imagecolor. A more quantitative approach is to use known wave-length sources. A neon lamp is a traditional possibility,although laser pointers are commonly marked with theiremitting wavelength, and datasheets for light-emitting diodestypically also indicate the peak wavelength and the spectralwidth.

Figure 5 shows image profiles of the spectra of severalLED sources. The total intensity, the simplest measurementto extract from ImageJ, is shown as function of the pixelnumber. Intensities are reported as data numbers (DN); mostimage formats report intensity in each color channel as an 8-bit integer. The position of the peaks, in terms of pixel num-ber, is readily measured using the profile tool in ImageJ orby importing the profile data into a spreadsheet, and is shownin Fig. 6 versus the wavelength of the light source. Thepoints in this instance, which represent the peak positions,are adequately fit by a straight line: y¼ 755-2.4x, where y isthe wavelength in nm and x is the pixel number at which themaximum intensity is found. The uncertainty in the wave-length obtained with this fit is 610 nm. Although a slightnon-linear behavior is observed in the calibration data, thestraight line fit is adequate for most basic applications. Thecalibration errors are comparable to the breadth of the spec-tral emission. Figure 5 shows that the peaks have widths of10–20 pixels. These widths represent a convolution of the

Fig. 5. Measured image intensities across the sensor for five monochromatic

LEDs. The diffracted peaks are seen towards the left, while the zeroth order

images appear between pixel number 260 and pixel number 350. In (a) the

red LED (625 nm) peaks around pixel number 50, the green LED (565 nm)

has two peaks at pixel numbers 90 and 98, and the blue LED (470 nm) peaks

at pixel number 123. In (b) the red intensity is shown again for reference,

the amber LED (592 nm) peaks at pixel number 70, and UV light (390 nm)

peaks around pixel number 145.

Fig. 6. Stated wavelengths of the laser and LED sources plotted as function

of the pixel numbers in the images at which intensity is a maximum. The

symbol sizes correspond roughly to the accuracy with which peaks can be

located, while the bars show the width at half-maximum. The line fit is

described in the text.

171 Am. J. Phys., Vol. 82, No. 2, February 2014 Apparatus and Demonstration Notes 171

size of the image of the fiber and the spectral emission curve.It must be noted that some sources have considerably nar-rower peaks than others.

IV. SPECTRAL SENSITIVITY

Absolute intensity calibrations are beyond the scope ofthis paper and likely beyond the needs of most users.Relative intensity measurements can be made, which showthe spectral sensitivity of the webcam pixels.

The relative intensity of the pixels can be studied by usingthe ‘Color! Split Channels’ function in ImageJ. This splitsthe color images into separate grayscale images showing theintensity of the red, green, and blue components of the web-cam pixels. These images can be analyzed separately usingthe image profile function. An example of the results of sucha study is shown in Fig. 7.

The shapes of the responses merit discussion. They are theproduct of the spectral intensity of the source and the spec-tral sensitivity of the pixels. While the zeroth-order image ispanchromatic, at each pixel position in the first-order spec-trum, the intensity is fixed by the brightness of the source atthat wavelength. However, the recorded intensity in eachcurve at that position is different, owing to the color filterson the camera pixels. In some cases, this causes rather com-plex shapes of the curves. One straightforward experiment toperform is to divide the intensity in one color channel by thesum of the intensities of all three channels, an operationreadily implemented in a spreadsheet. Except where theoperation is ill-defined due to very low signal intensities, theresults, shown in Fig. 8, are a good indication of the spectralsensitivity of the color pixels of the webcam. In this exam-ple, the colors are very impure: each of the three color chan-nels respond with similar intensities to light in the550–600 nm wavelength range. While the red and blue chan-nels dominate in the red and blue wavelength regions, theregion where the green channel is the strongest also containssignificant contributions from the red and blue channels.

V. CONCLUSIONS

A simple spectrograph has been constructed with a web-cam. Despite a low parts count and simple and rapid assem-bly from inexpensive parts, the system allows quantitativestudy of bright laboratory sources.

Fig. 8. The relative webcam pixel response for the RGB pixels, obtained by

dividing the measured responses by the sum of the intensities of the RGB

channels, plotted as function of wavelength. The results shown here were

obtained with an incandescent source. The near-equal response of all chan-

nels at green/yellow wavelengths (550–600 nm) is evident, as are the selec-

tive red and blue peaks.

Fig. 7. Intensities of the separate RGB channels of the color images for two narrow-band sources (a green laser pointer and a red LED) and two broad-band

sources (an incandescent lamp and a white LED). The measured intensities depend on the spectral intensities of the sources and the spectral sensitivities of the

individual RGB pixels of the sensor.

172 Am. J. Phys., Vol. 82, No. 2, February 2014 Apparatus and Demonstration Notes 172

ACKNOWLEDGMENTS

The author acknowledges the support of a JanneyFellowship from the Johns Hopkins Applied PhysicsLaboratory during the documentation of this work, whichwas originally performed with the support of NASA via theDiscovery Program “Titan Mare Explorer” Phase A Study.

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“Cell-phone-based platform for biomedical device development and educa-

tion applications,” PLoS One, 6, e1570 (2011).4G. C. Lisensky, R. Penn, M. J. Geselbracht, and A. B. Ellis, “Periodic prop-

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