A Low Cost LED Based Spectrometer

Tai-Sheng Yeh* ( ) and Shih-Shin Tseng ( )

Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan, R.O.C.

A low cost LED based spectrometer is described. This LED based spectrometer could be operated as a

standalone instrument or under PC control via serial link. A total of seven wavelength selections are avail-

able by the plug-and-measure LED light module. With the seven wavelength selections, the LED based

spectrometer could provide qualitative visible absorption spectra that predict the absorption maximum.

Based upon the qualitative visible spectra, quantitative photometric information could be obtained.

Keywords: LED; Spectrometer; USB 2000; Low cost; Microcontroller.

INTRODUCTION

There is a need in many resource limited countries

and laboratories for a low cost analytical instrument that

could provide both qualitative and quantitative chemical

information. There are also great needs for low cost auton-

omous instruments that could help monitor the fragile envi-

ronment around the world on a global scale.1,2 With the ad-

vances in modern electronics, microcontrollers could pro-

vide great capabilities to acquire and process sensor sig-

nals. The development of compound semiconductor manu-

facturing technology could also produce many LEDs with

wavelengths from IR down to the 365 nm UV range. With

its compact size, a wide range of selections of available

wavelengths and low power consumption, LEDs are used

extensively in analytical instruments as light sources.3-15

Dasgupta et. al. gave a very detailed review for LED appli-

cation in flow injection analyses.3,4 LED light sources for

sensor applications were also extensively reviewed by

Kostov et al.5-7 Hauser et al.8-10 built a multi-wavelength

spectrometer to analyze aluminum, copper, ammonia, cal-

cium, phosphate, chromium, and nitrile. In his work, LEDs

with seven wavelengths and a 2 � 7 fiber optic coupler were

used to construct a multi-wavelength spectrometer. The 2 �

7 fiber optic coupler would greatly increase the cost of the

spectrometer and would not be suitable for large scale em-

ployment. Hamilton et al.11 built a four color LED photom-

eter with a material cost of about US$ 200. The LED pho-

tometer was designed with analog circuits and thus was not

very flexible or user configurable. Knagge et al.12 built a

spectrometer with a mini light bulb, diffraction grating and

LEGO block. The material cost was also about US$ 200.

This spectrometer was not robust or inexpensive enough

for large scale deployment in the field. Cantrell et al.13 de-

signed a SLIM spectrometer with red, yellow, and blue

LEDs. The total cost to construct the SLIM spectrometer is

about US$ 25. A TSL230 programmable light-to-fre-

quency converter from Texas Advanced Optoelectronic

Solutions (TAOS) was used as photo-detector for the SLIM

spectrometer. The SLIM spectrometer is low cost, low

power consumption and robust, thus it is very suitable for

autonomous instruments employed in the field. The only

limitation is the available wavelength in the SLIM spec-

trometer.

This work intended to build a robust, low cost, multi

wavelength LED based spectrometer using plug-and-

measure LED light source modules. A TCS230 program-

mable light-to-frequency converter from Texas Advanced

Optoelectronic Solutions (TAOS) was used as the photo-

detector in the current design. The total cost of the present

design is about US$ 20. With a wide range wavelength se-

lection and low cost solid-state electronic components, this

LED based spectrometer is a very cost-effective instrument

for resource limited developing countries and laboratories.

EXPERIMENTAL

The Chemical Reagent and Electronic Components

The Universal Indicator of pH solution is purchased

from Panreac, Inc. The pH dye Bromocresol Purple and

Bromocresol Green were purchased from Lancaster Inc.,

and Phenol Red and Thymol Blue were purchased from

TCI, Inc. All pH buffer solutions were purchased from

Journal of the Chinese Chemical Society, 2006, 53, 1067-1072 1067

Panreac, Inc. The LED was bought from RS Components

and local parts vendors. The TCS230 sensor chips were

bought from TAOS, Inc. The PIC Micro 16F877A and

electronic components were sourced from local electronic

parts vendors.

The Circuit and Hardware Tuning

Fig. 1 is the circuit design for the LED based spec-

trometer. The TCS230 chip from TAOS, Inc. was used as

photo-detector. Output-frequency scaling and photodiode

color selection is set by a dip-switch connected to the S0,

S1, S2, S3 input pin on the sensor chip. The sensor fre-

quency output is received by a PIC Micro 16F877A micro-

controller via PORTC.2. The signal after processing by a

microcontroller is displayed on the LCD and transmitted to

a PC via a RS232 serial connection. Thus the LED based

spectrometer could operate standalone or be controlled by a

PC to have a better visualized view of the obtained data. PC

control software written in Visual Basic 6.0 was used to set

the operation mode for the experiment. The spectra of the

plug-and-measure LED light sources are shown in Fig.

2(a). The emission peaks for the LED light sources were

389 nm, 407 nm, 462 nm, 527 nm, 572 nm, 587 nm and 620

nm. The actual device hardware is shown in Fig. 2(b), 2(c),

2(d).

The LED light intensity and the emission bandwidth

could be adjusted by the current-limiting resistor. Because

the LED light intensity and the emission bandwidth would

affect the measured absorbance by the LED based spec-

trometer, calibration against a commercial spectrometer

with standard solutions is necessary for the LED based

spectrometer. The best empirical value for the current-lim-

iting resistor is set by fitting the absorbance spectra ob-

tained by a LED based spectrometer to that obtained by a

Ocean Optics USB 2000 spectrometer. A series of standard

solutions to calibrate the LED based spectrometer were

1068 J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 Yeh and Tseng

Fig. 1. Circuit for LED based spectrometer.

prepared by adding 5 �l Universal pH Indicator to 2 mL

buffer solutions from pH 1 to pH 13. The spectra for the 13

standard solutions obtained by Ocean Optics USB 2000

spectrometer are shown in Fig. 3(a). The obtained spectra

serve as reference for setting the value of LED current-

limiting resistors. The best value of the current-limiting re-

sistor was set by comparing the spectra obtained from a

USB 2000 spectrometer. Fig. 3(b) shows the best value for

a blue LED (462 nm) was 2 k�.

Experiment Procedure for Spectra Mode

Typical LEDs have FWHM values of about 25 nm

and a typical FWHM molecular absorption band is over 50

nm. Thus it is possible is use a discrete LED absorbance

measurement to approximate the absorbance spectra of mo-

lecular absorption.

The resistor value for each LED light source was de-

termined by comparing it with an Ocean Optics USB 2000

spectrometer. Herein the spectra of different buffer solu-

tions from pH 1 to pH 13 were measured, and the resistor

value was tuned to best fit the spectra obtained by the USB

2000 spectrometer. After the resistor value was fine tuned

for the LED based spectrometer, four indicator solutions:

Bromocresol Purple, Bromocresol Green, Phenol Red and

Thymol Blue were measured and compared again with the

USB 2000 spectrometer. This is the testing procedure for

testing the performance of the spectra mode.

Experiment Procedure for Photometric Mode

A series of the Universal Indicator solution from 1.25

�l to 200 �l is dropped to 2 mL pH 7 buffer solution. The

A Low Cost LED Based Spectrometer J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 1069

Fig. 2a. Spectra of LED light source.

Fig. 2b. Plug-and-measure LED light source module

and TCS230 detector module.

Fig. 2c. LED spectrometer with cover removed.

Fig. 2d. LED spectrometer with cover in measurement

mode.

absorbance variation caused by concentration difference of

indicator solution is compared with the USB 2000 spec-

trometer for the photometric mode.

RESULTS AND DISCUSSION

Spectra Mode

The absorption spectra obtained by the LED based

spectrometer are shown in Fig. 4. For the Bromocresol

Purple and Bromocresol Green dye solutions, the LED

based spectrometer showed very good qualitative agree-

ment with the USB 2000 spectrometer and provided a

good prediction of their absorbance spectra. In the spectra

region for the maximum absorption of Bromocresol Pur-

ple and Bromocresol Green, the LED light sources are

spaced closely. For the absorption spectra of Thymol Blue,

the LED based spectrometer didn’t predict the absorption

maximum very well. The reason was that in the present

spectrometer design there was no 430 nm LED light source.

For the absorption spectra of Phenol Red, the LED based

spectrometer didn’t produce very good absorption spectra.

This is caused by the large FWHM values of the blue LED

(peak 462 nm) and green LED (peak 527 nm). According to

Beer’s law, a polychromatic light source would give a large

deviation compared to a monochromatic light.14 The lack

of a good 430 nm LED light source plus the lack of a light

source between blue LED and green LED also contributed

to the ill-behaved absorption spectra.

Photometric Mode

Comparison of photometric performance with an

1070 J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 Yeh and Tseng

Fig. 3a. Spectra of pH buffer solution from pH 1 to pH

13 for Universal pH Indicator.

Fig. 3b. Tuning resistor to fit USB 2000 Absorbance.

Fig. 4a. Comparison of Bromocresol Purple spectral

data with a USB 2000 spectrometer.

Fig. 4b. Comparison of Bromocresol Green spectral

data with a USB 2000 spectrometer.

Ocean Optics USB 2000 spectrometer is shown in Fig. 5.

The LED based spectrometer showed a similar trend with

the Ocean Optics USB 2000 spectrometer for the full mea-

surement range. For the concentration range from 10 �l to

80 �l, the agreement with the USB 2000 spectrometer was

very good for the LED based spectrometer. The LED based

spectrometer could yield very good quantitative results for

absorbance values under 0.8. In daily practice, absorbance

measurements seldom exceed 1.0. Many commercial in-

struments also fail at high absorbance values. The devia-

tion of the LED based spectrometer in the high absorbance

region could be due to the polychromatic nature of the LED

light sources.14

CONCLUSIONS

Both spectra mode and photometric mode for the

LED based spectrometer had shown very comparable re-

sults compared to the Ocean Optics USB 2000 spectrome-

ter. With low cost electronic components, it is possible to

build a cost effective instrument for resource limited labs.

The electronic components for a LED based spectrometer

had a total cost of about US$ 20. Commercial instruments

usually would cost more than US$ 1,000. In this respect,

the LED based spectrometer was a very cost-effective in-

strument. Inclusion of a kinetic mode for a LED based

spectrometer is possible by rewriting the firmware in a PIC

microcontroller. This kind of LED based spectrometer will

be very useful for both autonomous environment monitor-

ing and educational purposes. The capability of the present

LED based spectrometer can be further extended by incor-

porating more LEDs with different wavelengths.

ACKNOWLEDGEMENTS

The authors sincerely thank the National Science

Council of Taiwan (No. NSC 93-2113-M-268-001) for the

financial support of this work.

Received December 15, 2005.

REFERENCES

1. Diamond, D. Anal. Chem. 2004, 76, 279A-286A.

A Low Cost LED Based Spectrometer J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 1071

Fig. 4c. Comparison of Thymol Blue spectral data

with a USB 2000 spectrometer

Fig. 4d. Comparison of Phenol Red spectral data with a

USB 2000 spectrometer.

Fig. 5. Comparison of photometric data with a USB

2000 spectrometer.

2. Sequeira, M.; Bowden, M.; Minogue, E.; Diamond, D.

Talanta 2002, 56, 355-363.

3. Dasgupta, P. K.; Bellamy, H. S.; Liu, H. G.; Lopez, J. L.;

Loree, E. L.; Morris, K.; Petersen, K.; Mir, K. A. Talanta

1993, 40, 53-74.

4. Dasgupta, P. K.; Eom, I.-Y.; Morris, K. J.; Li, J. Anal. Chim.

Acta 2003, 500, 337-364.

5. Kostov, Y.; Rao, G. Rev. Sci. Instrum. 1999, 70, 4466.

6. Kostov, Y.; Rao, G. Rev. Sci. Instrum. 2000, 71, 4361.

7. Kostov, Y.; Rao, G. Rev. Sci. Instrum. 2003, 74, 4129.

8. Hauser, P. C.; Rupasinghe, T. W. T.; Cates, N. E. Talanta

1995, 42, 605-612.

9. Hauser, P. C.; Rupasinghe, T. W. T.; Tan, R. Chimia 1995,

49, 492-494.

10. Hauser, P. C.; Rupasinghe, T. W. T. Fresensius J. Anal.

Chem. 1997, 357, 1056.

11. Hamilton, J. R.; White, J. S.; Nakhleh, M. B. J. Chem. Educ.

1996, 73, 1052-1054.

12. Knagge, K.; Raftery, D. Chem. Educator 2002, 7, 371-375.

13. Cantrell, K. M.; Ingle, Jr. J. D. Anal. Chem. 2003, 75, 27.

14. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of In-

strumental Analysis; 5th ed.; Harcourt Brace: Orlando, FL,

1998.

1072 J. Chin. Chem. Soc., Vol. 53, No. 5, 2006 Yeh and Tseng