A simple refractometer can be made from household items, but would never be accurate enough to use with coffee. Coffee is a dilute beverage, and our tongues are adept at noticing slight changes in its concentration. This means any device we use to ascertain concentration of a coffee has to be incredibly accurate, or it wouldn’t be very useful. The accuracy of components and the level of engineering that goes into a high quality handheld refractometer are staggering, but it helps to understand the general principles at work.
A light source shines photons through a lens towards a prism. The light then travels in a very predictable and repeatable manner towards the interface of the prism and the sample. The light doesn’t actually travel into the sample, it merely touches the glass-liquid interface and reflects back down towards the detector (see picture). You might recall the classic science lesson where a beam of light passing through a glass prism changes direction. This is a similar phenomenon called total internal reflection, where the steep angle of the light allows it to reflect backwards.
If the sample has a higher refractive index, the angle of reflection will be more acute, and vice versa. The linear detector receives the light somewhere along its length. By determining where the light hits the detector, the angle of reflection — and therefore the refractive index — can be calculated.
A simple refractometer used by fruit farmers to record the concentration of sucrose in their fruit is called a Brix meter. A ripe coffee cherry will only have about three drops of available liquid coming out of them. If this liquid is applied to the lens of a spy glass-style refractometer, a farmer can determine the level of sugar content in the mucilage taking this as a barometer of when to pick, usually between 15 and 25% sucrose. Farmers measure Brix in the mucilage, not the bean, so it won’t give us a direct measure of how sweet the cup of coffee will end up.