BPC300-C Photochromic Contact Lens Spectrophotometer

To perform a transmittance measurement, the carousel is first rotated to a clear aperture for a 100% reference measurement.

The light from the probe source is unshuttered and mechanically chopped to produce an AC signal of known frequency. The light is shone through the probe source monochromator, which selects a 5 nm bandwidth band. The monochromatic light is collimated and shone into the water bath, through the clear aperture in the carousel, into the integrating sphere on the entrance of the measurement monochromator tuned to the same wavelength band, and into the photomultiplier. The signal from this is recovered by the acquisition electronics. 

By scanning the source and measurement monochromators across the wavelength range of interest, the spectral reference photocurrent can be measured.

The carousel is then rotated to the lens to be measured, and the measurement is repeated. The ratio of the sample to reference measurements yields the transmittance of the sample.

In order to simulate the effect of sunlight illuminating the sample, the light from a Xenon bulb is filtered, attenuated, and focused on the entrance of a fibre bundle. This bundle is split and both ends collimated and shone onto the same locations on the sample. 

During measurements with the bias light active, the chopped probe signal and measurement monochromator prevent the bright conditioning light from affecting the measurement. 

While the conditioning beam is active, the probe and measurement monochromators can be tuned to a single wavelength to measure the transmittance at that wavelength as a function of time. Once stable the monochromators can be scanned across the wavelength range to measure the spectral transmittance of the activated sample.

The carousel is rotated to a photodiode position at various points in the measurement flow to measure the intensity of the conditioning beam, to assure that it is operating at the correct intensity.