What is a "light box"?

Light boxes are simple devices for experimenting with light and matter. They are robust and inexpensive, and can be used for a wide variety of tests. In the Procedures section, you will find methods for determining the concentration of light-absorbing chemicals, fluorescent molecules and light scattering particles.

Each light box has a light source, a sample holder, and a light detector. The light source can be infrared, visible, or ultraviolet light, depending on the assay design. The light detector typically produces a voltage output, but frequency or resistance can also be used.

Figure 1. Light from the light source passes through a filter (optional), through the sample, and then strikes a light detector. The light detector produces a voltage that is proportional to the incident light.

Solutions with higher concentration allow less light to pass through, leading to lower voltage output (left graph). The concentration of the absorbing substance is proportional to the absorbance, which is the logarithm of the ratio of the voltage produced by a clear solution (Vo) and the voltage produced by the sample (V). The concentration of a light-absorbing solute in a test sample can be calculated by measuring the amount of light absorbed by the sample and comparing it to a standard curve made using solutions with known concentrations.

The geometry of the device can be changed so that the detector is at a right angle to the path of the light beam. This is useful for measuring fluorescence and light scattering.

Terms and definitions

Light box - A simple photometer.

Photometer - An instrument for measuring light absorption, light scattering, or fluorescence.

Solution- A liquid that contains a solute dissolved in a solvent.

LED - A light emitting diode.

Filter - Thin colored plastic strip. Used to modulate the color or intensity of light. Each filter has well known spectral characteristics.