A blast from the past: An Evelyn Colorimeter

For the past several years I have been working on a manuscript for a book on the use of colorimetric methods for the quantification of organic molecules. One of the major landmark pieces of equipment that I kept reading about was the Evelyn Photoelectric Colorimeter; it is basically a quaint little wooden box that put an affordable spectrophotometer into laboratories the world over. This was one of the pieces of essentially ended the days of comparators (devices in which you compare the color intensity of a sample to a reference material) which were often subjective and contributed semi-quantitative work at best. I myself used a comparator which used long Nessler tubes for comparing the color of liquid samples, which a bit of skill it isn’t so bad although having an unbiased eye was not always easy.

The Evelyn Photoelectric Colorimeter was the handywork of Kenneth Evelyn, hence the name of the device. Kenneth Evelyn published a paper in the Journal of Biological Chemistry title “A Stabilized Photoelectric Colorimeter with Light Filters” in 1936 (Evelyn, 1936; which can be viewed here). The design, while simple by today’s standards, was a major step forward in the field of spectroscopy. Perhaps unsurprising given the truly ground-breaking nature of this instrument, Kenneth Evelyn filed a patent in 1938 (United States Patent 2193315) for this photoelectric colorimeter for “biological” work. Consisting of a photocell and drop-in colored filters, the device put a cost-effective colorimeter into reach of many laboratories, ranging from industrial labs to clinical settings. Prior to this, a lot of chemical analysis consisted of traditional “wet” chemical methods such as titration, gravimetric methods which involved forming a precipitate and weighing the amount of material recovered, or the aforementioned color comparison methods in the event of a reaction that forms a chromophore.

Anyway, Kenneth Evelyn and a coworker (Helga Tait Malloy) published a paper on the colorimetric determination of Bilirubin using this device in the Journal of Biological Chemistry in 1937 (link) which is regarded as one of the landmark papers in clinical chemistry. A flood of other highly cited paper quickly followed which quantified a wide range of molecules using colorimetry including a high cited method for determining phosphorous published in the Journal of Laboratory Clinical Medicine.

A bit about spectroscopy and how we use light to measure stuff

The basic principle behind colorimetry, and spectroscopy in general, involves the principle of absorption and transmittance. Some types of molecules absorb light very strongly at specific wavelengths allowing their concentration to be measured if you can measure the amount of light transmitted through a sample at a given wavelength. The amount of light that is not transmitted to the sample due to absorption can be used to infer the concentration of a particular analyte. For those of you that have taken a course in college chemistry or biology have probably heard this referred to as Beer’s law, or more accurately the “Beer-Lambert law”, which is described by the equation:

Where A is the amount of absorbed light, I0 is the light intensity shining into a solution and I is the amount of light that makes it through the solution. So the cloudier the solution, the less light make it through or is absorbed. As you might guess, the amount of light absorbed by a solution is dependent on a few things, namely the concentration of the stuff that does the absorbing, the wavelength of light, and how “absorby” a particular thing is.

In this equation, A is again absorbance (at a specific wavelength), ε (epsilon) is the molar extinction coefficient (basically a measure of how “absorby” a particular molecule is for a specific wavelength of light), l is the pathlength (usually 1 cm to make the math easier!), and c is the concentration of a particular molecule. The beauty of this is that if we can measure the absorbance of light through a solution, we can infer the concentration of a particular molecule that absorbs at this wavelength! This technique has been widely applied to all sort of molecules over the past 100 years, from protein in blood and urine, to the amount of oxygen in the blood stream bound to hemoglobin, and literally tens of thousands of other kinds of molecules!

On the “absorby-ness” of a particular molecule as a function of the wavelength of light, consider carbon dioxide (which is something like 400 ppm in the atmosphere at the time of writing). Carbon dioxide gas lets visible light through with no problem, yet, infrared light is absorbed. If we could see infrared light, the world around us would appear to be very dark given how “absorby” CO2 is for light in this part of the spectrum!

The Big Find

About two or three years ago I stumbled upon the Science History Institute’s page where they have a photograph of an Evelyn Colorimeter (link here) and I have to say that I was taken back by the elegance of the design. There is something about the appeal of a wood box. And one that does science? I´d love to have one especially given my love of spectroscopy. Unfortunately, I figured such a think would be well out of my reach if such things are even still floating around out there!

In the spring of 2021, I stumbled across an Ebay listing for an Evelyn Colorimeter for $150 (USD). I could not believe the price for such an amazing artifact, and one in seemingly in fairly good condition given its age. Before I could bat an eyelash, I had my credit card number entered and a confirmation email in my inbox.

The arrival and a closer inspection

Upon receiving the Evelyn Colorimeter (Figure 1), the first thing that struck me when I opened the box was the unmistakable aroma of antique wood. It smelled absolutely fantastic. The potent yet gentle scent of the wood reminded me of my grandmother’s walk-in closet in her bathroom. Lifting the instrument out of the box, I was amazed at how sturdy it felt. The feel of 70+ years of grime on the surfaces was unmistakable. All things consider, I’m nothing short of amazed at how well this instrument has stood up against the passage of time.

Figure 1 – The Evelyn Photoelectric Colorimeter

As can be clearly seen from the name plate on the front, this instrument was manufactured by Rubicon in Philadelphia. Later iterations of this instrument were made by Honeywell (which still exists today).

The top of the instrument, home of various knobs and switches, is a stainless steel plate (Figure 2). On the left, we find the switches that control the lamps for the “galvanometer lamp” and “colorimeter lamp”, and the middle hosts a tube for slide a sample in with a slot for the desired wavelength filter (more on that later), and on the right we´ve got course and fine adjustment knobs.

Figure 2 – The top of the Evelyn Photoelectric Colorimeter

On the top metal plate, there is a hand etched inscription that reads “149-11-9” (which can be seen a bit more easily in Figure 3 below). I am not sure what significance these markings are but I have to wonder if this particular instrument was manufactured in 1949.

Figure 3 – The top panel is etched with several numbers etched at the bottom center

Figure 4-6 –

Figure 4 (left) – The switches on the left side of the instrument panel definitely have some wear after some heavy useage

Figure 5 (center) – The sample holder and filter slot

Figure 6 (right) – Even more grime!

The Evelyn Colorimeter that I purchased only came with a single filter (660/690 nm) although the seller displayed a 565 or 585/640 nm filter holder and a few other filters that didn’t make it to me for some reason (perhaps they got lost when customs opened up the box and inspected the contents?). Careful inspection of the Science History Institute’s images of this instrument reveals that the colorimeter came with a 420/515 nm and 540/600 nm filters. The slot holder in the back has 6 slots suggesting that this instrument may have originally came with more filters which have no doubt been last over the years.

Figure 7 – The rear filter holder of the Evelyn colorimeter

Based on the wear pattern on the back of the instrument, it is pretty clear that some filters (or at least specific filter slots) were used more than others. Each filter set comes fitted with two glass filters (such as the 660 nm filter seen below in Figure 8) which serve to filter the light from the source to a narrower set of wavelengths pass through the sample and reaching the detector.

Figure 8 – The 660/690 nm filter that came with my slightly used Evelyn Colorimeter

Opening the front panel, which has a very satisfying feel to pulling on the Bakelite handle, reveals the colorimeter´s internal components: a detector, a sample holder, the filter holder, and the light source (Figure 9). To my surprise I actually found an extra bulb in the internal compartment!

Figure 9 – The internals of the Evelyn Colorimeter include a detector, sample holder, and light source. Note the extra bulb holder assembly (complete with broken bulb) and an extra bulb in the holder at the right. White residue and damage to the bottom wood panel are apparent.

On the inside of the door, this is a pair of instructions (Figure 10) for the operation of the colorimeter. The paper is old and very discolored but otherwise intact. The piece of plastic material covering this paper has become very brittle with age. Removing the pins such that I can remove the paper and restore the wood underneath is going to be a chore.

Figure 10 – A scan of the original instructions for the Evelyn Photoelectric Colorimeter

Restoration plans

Over the coming weeks, I plan on restoring this beautiful piece of history to its former glory. I’ll be quite curious to see if I can safely remove the grime and repair some of the superficial damage that this instrument has endured over the decades.


Evelyn, K. A. (1936) ‘a Stabilized Photoelectric Colorimeter With Light Filters’, Journal of Biological Chemistry, 115(1), pp. 63–75. doi: 10.1016/s0021-9258(18)74751-0.

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