Review of Low-Cost / DIY Spectrophotometers

The development of low-cost and home-built spectrophotometers has been an active area of interest in the educational and scientific community for several decades. The motivations behind these efforts are clear: commercial instruments are often prohibitively expensive for schools and smaller institutions, and even when available, they offer little insight into the physical principles of the measurement. A wide variety of designs have been proposed over the years, differing in complexity, cost, target audience, and performance. This section reviews the most relevant examples from the literature, with particular attention to the design choices made and the lessons that can be drawn for the present work.

One of the earliest and most accessible designs is that described by Lema et al. [5], who built a spectrophotometer from components readily available to anyone: a halogen flashlight as the light source, a compact disc (CD) as the diffraction element, a light-dependent resistor (LDR) as the detector, and a standard digital multimeter for readout. The entire assembly was mounted in a cardboard box, with two simple slits used to collimate the diffracted light onto the detector. Despite its extreme simplicity, this device was capable of producing recognizable absorption spectra and calibration curves for several biochemically relevant substances, including chlorophyll, starch, and proteins. The authors demonstrated its use in a series of teaching experiments in a biosciences course, showing that meaningful quantitative results could be obtained even with such minimal means. The main limitation of this design is its poor wavelength resolution (rotating the CD by a single degree corresponds to a wavelength change of approximately 25 nm) and the fact that it provides resistance readings rather than direct absorbance values. Nevertheless, it remains a compelling demonstration of how far ingenuity can go in replacing expensive equipment.

A more structured approach was taken by Knagge and Raftery [7], who designed a LEGO-based spectrophotometer for use in a graduate-level chemical instrumentation course at Purdue University. Their instrument incorporated a miniature filament lamp as the light source, a convex lens and an infrared filter for light conditioning, a 1000 lines/mm transmission diffraction grating, a spring-loaded rotating mirror for wavelength selection, a silicon photodetector, and an amplifier circuit connected to a digital multimeter. The use of LEGO bricks as optical mounts was motivated by their precise manufacturing tolerances, modularity, and mechanical stability, which make them well suited to holding optical components in alignment. The instrument achieved a wavelength resolution of approximately 8 nm and was used to record absorption spectra and calibration curves for potassium permanganate. Compared to a commercial Spectronic 20 instrument, the home-built device showed higher stray light levels and a higher limit of detection, but its calibration curves displayed similar linearity. The total cost was approximately 200 USD. The authors emphasized that the educational value of the device lay not only in its use, but in the process of construction itself, through which students gained practical experience with the optical and electronic modules that make up any analytical instrument [7].

Building on the LEGO concept but targeting a much younger and less specialized audience, Albert, Todt, and Davis [8] developed a significantly simpler and cheaper LEGO spectrophotometer designed for use in high school physics and chemistry classes. Their design used a white LED as the light source, a single lens for collimation, a 1000 lines/mm slide-mounted diffraction grating, and a photodiode mounted on a rotatable arm for wavelength selection, with a digital multimeter as the readout. The entire apparatus was built on a standard LEGO baseplate and could be assembled and optimized in approximately 45 minutes. The total cost was around 25 USD, roughly one eighth of the cost of the Knagge and Raftery design. Despite its simplicity, the device produced absorbance measurements in close agreement with those from a commercial Ocean Optics USB2000+ spectrophotometer over the typical working range of absorbance values up to about 1, and was capable of recording full visible absorption spectra between approximately 420 and 660 nm. The authors noted that the use of familiar toy materials made the activity engaging for students, and that the transparent design effectively exposed the physical principles that are hidden in commercial black-box instruments [8].

A very different approach is represented by the work of Wang et al. [4], who demonstrated a smartphone-based spectrometer for colorimetric biosensing applications. Their device used the built-in LED flash of a smartphone as the light source, a compact disc fragment as the diffraction grating, and the smartphone's CMOS camera as the detector. The entire optical assembly was compact and required no external components beyond a simple sample cell. The smartphone camera captured the dispersed spectrum as an image, which was then processed using a dedicated Android application to extract the absorbance spectrum. The device was tested for the detection of glucose using an enzymatic colorimetric assay and for the detection of cardiac troponin I using peptide-functionalized gold nanoparticles, achieving limits of detection comparable to those of commercial bench-top instruments. While this design is more oriented toward portable point-of-care diagnostics than classroom education, it illustrates the remarkable potential of consumer electronics as spectroscopic platforms, and it highlights the growing role of smartphones and digital cameras as detectors in low-cost spectrophotometry [4].

González-Morales et al. [2] presented a portable low-cost UV-Vis spectrophotometer specifically designed for field measurements of environmental contaminants, with a particular application to the detection of mercury ions in water using a synthesized chemosensor. Their instrument operated in the visible range from 400 to 700 nm and used a 1 W white LED as the light source, a DVD-derived diffraction grating mounted on a stepper motor for automated wavelength scanning, and a TEMT6000 phototransistor as the detector. The entire device was housed in a 3D-printed black enclosure and controlled by an Arduino microcontroller, which performed the wavelength sweep, acquired the detector signal, computed the absorbance, and transmitted the data to a computer. Two independent sample chambers allowed simultaneous measurement of the sample and a reference, improving stability. The total cost of the device was below 86 USD. Absorbance measurements agreed with those from a commercial Cary 60 spectrophotometer with error percentages below \(10\%\) for most concentrations, and below \(5\%\) for the mercury detection experiments. This design is particularly noteworthy for its level of integration and automation, demonstrating that a microcontroller-based instrument can approach commercial performance at a small fraction of the cost [2].

Finally, the Viking spectrophotometer developed by Kolesnichenko et al. [6] represents one of the most pedagogically sophisticated examples in the recent literature. Rather than aiming for maximum analytical performance, this design prioritized the learning experience above all else. The instrument was built from wooden holders, inexpensive magnifying glass lenses, a white LED light source with a replaceable aluminum foil slit, and a transmission diffraction grating mounted in a slide frame. Crucially, no fixed housing was used, all components were freely positioned on a flat table, allowing students to experiment with different configurations, observe the effects of misalignment, and develop genuine physical intuition. The spectrum was projected onto a white cardboard screen and photographed with a smartphone camera for digitization. The device supported both absorption and fluorescence measurements and was successfully deployed across a wide range of educational levels, from high school outreach activities to Master and PhD level courses at Lund University. Its total material cost was approximately 14 euros per set. The authors reported that engagement with the Viking spectrophotometer measurably improved the quality of students' laboratory reports and their confidence when subsequently operating professional instruments [6].

Taken together, these examples illustrate that low cost and high educational value are not mutually exclusive, and that the integration of a microcontroller with dedicated software can dramatically extend the functionality of a home-built device without greatly increasing its cost. These considerations have directly shaped the design decisions described in the following chapter.