The beam of light from a projector passes through a variable slit and lens to a diffraction grating, from which it is reflected to a screen, where it appears as a color spectrum. A solar cell connected to a voltmeter that is connected to a large readout provides a measure of the intensity of the light incident on the screen. As you move the solar cell across the spectrum on the screen, the voltmeter reading varies with the intensity of the light in the different regions of the spectrum.

White light projected through a variable slit and lens to a diffraction grating is reflected to a screen as a visible color spectrum. A photocell connected to a voltmeter with large readout is moved across the screen into and out of the visible spectrum. The voltmeter readings show not only the different intensities within the visible spectrum, but intensities outside the visible spectrum as well. Due either to frequency response of the photocell or blockage of UV light by the glass of the projector, the readings are negligible in the UV part of the spectrum.