Introduction: An Inexpensive Photometer and Colorimeter
What is this instructable about?
The aim of this project was to construct a simple and inexpensive device that allows the measurement of the color composition of a solution, i.e. a colorimeter, and/or its optical density, i.e. a photometer.
It is intended be an educational tool for science classes, but it might be also be used by hobby scientists, citizen science groups, and probably for a multitude of other purposes.
To read the data from the sensors, the unit it may be either be connected directly to a Raspberry Pi or, via an Arduino, to a PC.
The estimated total cost for all parts required are in the range of 30 € or US$ (w/o shipping costs).
As you can use a Raspberry Pi Zero or Arduino Nano, the additional costs can also be very limited.
The layout of the housing allows to build both photometer and colorimeter from the same parts, just depending on the sensor breakouts and LEDs you will use.
So what can you do with the device?
A photometer allows to measure the amount of light reaching a sensor. Given you are using a constant light source, and you place a translucent object between light source and sensor, this allows you to calculate the amount of light that gets lost on this way. This defines the optical density of the object. The "lost" light could either be absorbed by a dye, or scattered by particles, or a combination of both. So a photometer allows to quantify the optical density of a clear solution, e.g. some ink, or a suspension, like milk or yeast particles in beer, wenn filled into a standard photometer cuvette.
A colorimeter allows you to measure the color composition of an object, here of a solution. The sensor used here for the colorimetric device splits the visual spectrum into three channels, red, green and blue (RGB), hereby simulating the way our eyes see colors. For most colors this allows to describe our impression of a color by a combination of three numbers describing the intensity in the three RGB channels, typically defined in 256 steps. So black is 0-0-0, white is 256-256-256, pure red 256-0-0 and so on. Pure yellow would be 256-256-0, pure cyan 0-256-256. But the sensor used here is very precise and allows to discriminate over 67,000 steps for each color channel.
Photometers are often used to measure chemical reactions, like changes in the acidity (pH) of a solution, by measurement of the intensity of an indicator dye. For optimal detection you may want to measure only the absorption of light close to the absorption maximum of the dye. Therefore you will prefer to use a monochromatic light source instead of white light. LEDs come in a wide variety of colors, give nearly monochromatic and intensive light with a very stable intensity pattern and are very energy effective. The sensor used for the photometric layout allows you to measure light intensity over a very broad spectrum, from UV to IR, with a very high precision.
A variety of indicator dyes are changing their color, e.g. from yellow to blue. Here a colorimetric measurement can allow to demonstrate and quantify this transition process, as the colorimeter described here could also be defined as a three-channel photometer.
So you may for example measure the pH, oxygen and CO2 concentrations in your pool or aquarium, or nitrate and lead concentrations in your drinking water. Or just analyse the precise color of your favorite ink, so you may paint your flat in the same color.
As the costs per unit are extremely low compared to any commercial devices, it is a nice tool for science classes and hobbyists.
Step 1: Basic Layout of the Device; Required Parts
The device consists of three core elements:
- the housing and lid are made by assembly of pieces that were laser-cut from a single sheet of 3 mm acrylic (see pictures next step). They are defined by a single SVG-file. I am using the laser cutting service of Formulor (Berlin/Germany, 13€ per plate plus shipping), but using the same SVG-file, the parts could also be ordered at Ponoko (US) or RazorLAB(UK) at a similar price. You may choose 