A virtual spectrophotometer

A spectrophotometer directly measure absorbance A and its reciprocal 1/A, the percent transmittance or %T.

It contains a light source, a diffraction grating to select the wavelength of the light to be used, a place for the sample, and a photodetector.

The photodetector responds to light by generating an electric current.

When there is no sample in the machine, light to the photodetector is blocked by an occluder. There is noise in the electrical system that gives rise to a small dark current that must be corrected for.


When a sample is inserted, the occluder is moved out of the way.

Light, passing through the specimen reaches the photodetector. A current, proportional to the intensity of the light, leads to the movement of a needle on a scale.

Walk your way through the use of the vSPEC using this animation

Next you will use the working model to make observations.

  • Do A and %T measure the same or different properties of a solution?
  • Why do you "blank" and "zero" the vSPEC?  What does that control for?

Use Wikipedia | revised 19 March 2005