When Analog Computers Ruled the World

Boy that reads like science fiction! This is actually a nonfiction story about my experience with a type of analog computer that was alluded to in a thesis by Mr. Richard S. Hunter and found by a scientist at Procter & Gamble who just had to have one. Actually he couldn’t have one as the crafty Mr. Hunter insisted that it would take an order of 25 to make it worth his while to set up a development project hence the machines were called the D25.

D25 (Analog Computer)

The machine was designed in 1956 and remained in production for many years, after that they were kept running with parts salvaged from retired machines. The D25 measured colors, it did this by producing a specific illumination and focusing it on the specimen then it then gathered the reflected light looking for what was missing. The missing light represents whatever the specimen absorbed. The model pictured above is from the original 25 and is said to be still working.

By analyzing the difference between the light you shine and the light you record can give you certain information that can be used to give numbers to what you can see. This is not an absolute method which is why the machine is called a color difference meter. Inside the heart of the machine was four photomultiplier tubes, each tube was behind a different filter. Those filters are referred to as X,Y and Z plus Xa, Xa was added later.

Analog computers need constant calibration, lamps fail, voltages fluctuate, tubes age, filters solarize, even the temperature causes the readings to change. This was one of my first jobs keeping these old kunkers going just long enough for the company to allocate enough money to buy a newer one. The newer one was also called the D25 but to differentiate it from the tube behemoth the suffix “-2 solid state” was added.

Hunterlab D25-2Δ
(I searched hard but could not find a photo of this machine, If I find one I’ll replace this)

Both models were essentially the same machine, they had to be as by then industries all around the world had written specifications that required that the new one read the same as the old one just smaller, lighter and without the vacuum tubes.

Q: BUT HOW DO YOU READ COLOR? Well I’m glad you asked. The machine does not read the whole color but looks at specific areas and that has been determined to give enough data to make certain assumptions about the color. The real purpose of the machine is to tell how far off the ideal color you are and not the absolute color, that’s a much easier job.

tristimulus color space

Above you can see the four filters of the tristimulus colorimeter. Calibrated plates plus a zeroing mechanism or standards are used to make the readings equil the standards. Once done you can read the percent reflectance of light from your sample. Percent reflectance X,Y,Z does let you write down the numbers but tells little about what color you are looking at.

Mr. Hunter’s brilliant idea

If you are familiar with tools like Photoshop you may have noticed next to the crayon color picker there’s a scale called L,a,b. This is one of Mr. Hunter’s greatest contributions to the world of color. He figured out a way to take the XYZ numbers and output Lab. Lab is great because for the first time you can look at the number and have an idea of what the color is. L is the value of light from very dark to very light, a is the shift from green to red going from negative to positive and b is the shift from blue to yellow. This is called the opponent color scale and works similarly to your eyes as you can see a reddish green or a yellowish blue.


XYZ to Lab

Here’s where the analog computer of the two machines comes in, you take the voltages coming out of the photomultiplier/germanium photocell represented by the formula above and apply that math and your results will be in the appropriate voltages. In the early machine you had to turn a twenty-five turn dial until a meter went null and take your reading off the dial for each reading. On the more modern D25-2 you pushed a row of radio buttons and read the value off a volt meter one reading at a time.

Digital Color Measurement

If you’re interested you can reproduce this experiment if you have a color image editor like Photoshop. Open a photo you took, find the color picker and set the scales to XYZ and Lab. Hover the picker over the color in the image you want to see the tristimulus value for and look in the info panel to see your results. That’s it! If you want to calibrate your image make sure there is one know color standard is in your photograph and use the Image -> Color balance to adjust your standard then read your sample. Wasn’t that easy?

The computer had a special board with a very expensive operation amplifiers that would output a voltage that was the square root of the input voltage, others were multipliers and dividers. These machines lasted a very long time in the field and as they aged many of them could no longer be fully calibrated but this mattered little as their intended purpose was as a color difference meter so we could calibrate them to the customer’s standard and they only had to measure the small difference their production was varying from their standard.

My calibration helper

When I started to calibrate analog computers in 1983 I used this calculator to make sure my conversion results were correct for the range of colors I was working with. I still have this calculator and I had it a few years earlier when I took electronics at college. As you can see you can program in formulas, slide the switch and hit the COMP button and it would query you for XYZ and produce Lab so I didn’t have to carry around any tables. Of course I was holding in my hand what would eventually end the work I was doing.

HunterLab Standards


Above is a photo of my personal set of standards, given to me by my good friend and mentor Randy Bohman when I left after ten years of selling, servicing and installing HunterLab equipment and software. This was an amazing experience to not only travel all over Canada and parts of the world but to meet and work with so many leaders in the field from innovators like Mr. Hunter and Mr. Harold to all the color scientists working in the industry and sitting in on the development of technical standards to be invited into the labs, studios and testing areas of every industry imaginable.

By the late 1980s these machines were heading to the junk pile when the D25-9 was introduced. It could display all the readings at one time on its fluorescent display and used an analog to digital converter that eliminated the need to carry around tables to figure out what was going on. The source/sensor was changed little for the new model.

D25-9 M sensor

The D9 as we called it was a terrific machine and had options out the wazoo like the printer you can see in the photo above. It also had a row of hidden buttons that could contain options like extra memories or color scales. The tomato industry had a special button and so did a number of P&G products, there was also a hidden button that accessed the diagnostics.

This new wonder machine ran on a somewhat propetitery card bus. This was sort of an industrial standard for plug in cards, on one card was an 8088 processor and another card had the ROM and another the RAM, there was a card to run the printer and one to run the display plus a serial card for external communication. The A/D converter was on its own harness and connected directly to the CPU via a wide ribbon cable. Other than the photocell system this was an all digital machine, the end of analog computers at Hunter Associates Lab and the eventual move to spectral data.

Make sure you read my post of meeting and working for Mr. Hunter. Two Color Pioneers and a Kid from Toronto


3 thoughts on “When Analog Computers Ruled the World

  1. Hi Brian;

    I ran across your “When Analog Computers…” article today while doing some research on some of our old instruments here. We have three Hunterlab D25Ms with the D25-9 processor, and we have a customer who may be interested so I was trying to answer some of his questions.

    I haven’t pulled the instruments down or powered them up yet, and it appears that nobody else has either since they arrived here in Mrkham back in 2004.

    Do you have any advice to offer, or things to watch out for, when we power them up?

    We also have a DP-9000 processor that came in with a different instrument (since scrapped). Do you know if it is possible to interface an older D25 with the DP-9000? If so, what cable connections would be needed?

    Nice to read your story; hope you don’t mid me picking your brains & perhaps bringing back some memories.

    Best regards;

    Richard Hennick

    1. Oh that’s funny, these machines would have been from my patch. If they still have calibration tags I used to sign with “B2” or “B. Boni”. How long have you been with Labequip? We bought off you and I’ve done work for you too, I remember repairing some power supplies and going over some Hunter gear, last time was likely around 2006 might have been a LabSCAN II.

      As far as powering them up I don’t think there is any special technique if you mean a “soft start” as you know these have to run 24/7 so if something blows better it blows for a tech than in the field. I’d make sure the optics are clean to keep the lamp from burning any debris that may have fallen in, also check things like heat filters are good and in place. Some of the optics were bonded, I wonder how that stuff has lasted.

      We’re talking D9 and newer, you can swap sensors if the cables are the same, or you can make adapters from Canon to Amp plugs for the early ones, those are handy to have around. The machines won’t have the right calibration data but you can just program them in manually and use an alternate calibration standard. Most people are going to use these machines as colour difference meters and will have their own target standard anyway so I shouldn’t be a big deal.

      You’re welcome to get in touch with me if you like.

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