Patent GB-1302196-A (GB)
IPC: G01J3/51
Filed: 1969-04-23
Abstract: 1302196 Photo-electric absorption spectrophotometer AGFA-GEVAERT 2 April 1970 [23 April 1969] 20766/69 Heading G1A In a spectro-photometer which measures the absorption of a layer 20 by comparing its attenuation of a measuring wavelength which is strongly absorbed thereby with its attenuation of a reference wavelength which is substantially unabsorbed, a reference signal is provided from a photo-cell 22 when a reference filter 25 of a rotatable filter disc 16 is in the radiation path whereas a slowly increasing measuring signal from photo-cell 22 is provided as the filter 28 moves through the radiation path, the absorption of the layer being indicated by the angular position of the disc 16 when the measuring signal is equal to the stored reference signal. To measure the absorption, and hence the thickness, of a water-containing coating on a web 20, the reference and measuring wavelengths are 1À71 Ám and 1À96Ám respectively. The absorption of the layer is related to the amount of radiation scattered or diffusely reflected from the coating specularly reflected radiation being intercepted by mirror 14. Further filters 13, 23 limit the radiation from a source 10 to the IR region of interest, and the detector is a lead sulphide cell. The intensity variation in the measuring signal is achieved by super-imposing on the filter 28 an optical wedge 29 whose density changes as indicated. The resulting output signal from the detector is shown in solid lines in Fig.3. The storing of the reference signal and the comparison of the reference and measuring signals are controlled by pulses generated by a magnetic disc 18, rotating at the same speed as disc 16, passing a magnetic pick-up 19. While the reference filter 25 is in the radiation path, these pulses operate a monostable multi-vibrator (34), integrator (36), Fig.6 (not shown), gate (33) and flip-flop (40) to divert the detector output to a holding circuit (43). When the measuring filter 28 enters the radiation path the pulses are registered on a counter (48). Counting stops when the measuring signal equals the stored reference signal, and the count is a measure of the absorption of the sample. The counter is re-set for each revolution of the filter disc. In a further embodiment, Figs. 5, 7, 8, (not shown), the reference beam comprises the wavelength band 1À79 to 2À17 Ám isolated by the filter 13 (23), In this case the 1À96 Ám measuring filter occupies the smaller area 25 (50), while the area 28 comprises a neutral grey filter. This grey filter cuts down the reference beam intensity to the extent that the presence in this beam of the absorbing 1À96 Ám component does not lead to a significant error. The detector receives radiation that has passed through the sample, and the measuring signal is stored Fig. 5 (not shown), and subsequently compared with the reference signal as in the previous embodiment. A meter calibrated directly in thickness units may be provided instead of the counter. The incandescent lamp source 10 and detector may be positioned above a moving strip of material to measure its longitudinal profile. Or they may be scanned together across the moving sheet to provide a lateral profile.