The apparatus allows a spectroscopic measurement of the energy gap in semiconductors (Ge, Si) at room temperature.
The internal photoelectric effect (photon absorption and electron-hole pair production) is excited by monochromatic light, whose wavelength is continuously changed by rotating a reflection diffraction grating and modulated by a chopper.
Both the signal due to absorbed photons (photoconductivity) and the signal produced by photons transmitted through the thin semiconductor sample are counted simultaneously by a two-channel lock-in amplifier driven by the chopper reference signal.
The grating rotation is driven by a motor and the working wavelength is calculated from the rotation angle measured by a built-in encoder.
The absorption spectrum is obtained from the sample resistance changes, and the transmission spectrum from the light measured by a pyroelectric detector. A measurement without sample provides the emission spectrum of the light source (incandescent lamp).
A PC may be used as controller, data-storage and data-handling systems.