White light projected through a variable slit and lense 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.