Typical daylight is made up of light of all the colors of the spectrum: red, orange, yellow, green, blue, and violet. The human retina is sensitive only to the visible light spectrum. This visible light spectrum is a narrow band in the electromagnectic spectrum that includes high energy radiation such as x-rays and ultraviolet rays, and low energy radiation such as infrared rays and microwave rays.
When all the wavelengths of the visible light spectrum strike your eye at the same time, white is perceived. The sensation of white is not the result of a single color of light, but rather is the result of a mixture of all colors of light. (Technically speaking, white is not a color at all.) If all the wavelengths of the visible light spectrum give the appearance of white, then none of the visible wavelengths of light would lead to the appearance of black. Therefore, just like “white,” “black” is also not actually a color.
When white light strikes a solid, opaque object, two things can happen: it can be reflected or it can be absorbed. If all the light is reflected, the object will appear white. If all the light is absorbed, the object will appear black.
So, what makes something appear red? An object will appear red under white light because it reflects the red portion of white light, and absorbs all the other colors. For example, lycopene, the chemical produced by ripe tomatoes, absorbs most of the visible light spectrum, but reflects red light. That’s why ripe tomatoes appear red. If that same tomato were illuminated with a blue light, it would appear black since there was not red light to reflect.
How does all this relate back to colorimetric tests? First let’s look at the color spectrum produced by a few different types of light sources:
Incandescent lamps give off a broad color spectrum, similar to natural daylight. Fluorescent lamps give off only specific wavelengths of light, and have a high proportion of blue and green. High pressure sodium lamps give off a lot of yellow light.
Now, let’s look at stuff your butcher has known for quite a while. Here’s a picture of different types of meat illuminated with different types of light:
Incandescent bulbs can be modified to influence how meat looks in the display case to give the most desirable appearance. High intensity discharge lamps give off an orange cast and make the meat look yellow or blue, like the top row of colors, and fluorescent lights give off a lot of blue and green light, and make meat look more gray.
When using colorimetric tests, the same types of things can happen. The inks used to produce the reference color chart are made up of multiple ink/dye chemicals, and look a certain way when illuminated by a given light. The specific chemical produced for example in a chemical strip test, is only one chemical, reflects light different from the inks in the color chart. Depending on the type of light used to read the test, the strip can be redder, bluer, or grayer, and can even reflect the modified lighting in such a way that you can no longer differentiate between different test results.
All of the above has been taken into consideration when a colorimetric test is developed. Always follow the manufacturer’s directions!