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Optics
Fibroptonic
Ref : 202 012
ENGLISH
2.3 Observations and results:
•
We observe that the resistance of the photoresistive cell varies when it is moved
in the 1
st
order spectrum. In blue-violet, the resistance is high because the
photocell is less sensitive in this wavelength range and the light emitted by the
projector's halogen lamp has very little blue-violet radiation. The resistance is
minimal in yellow and orange (figure 25). Note the sensitivity of the photoresistor
to near infrared radiation (760 to 800 nm).
•
The curve no. 25 is a result of the combination of the spectral response of the
photoresistor and the spectral emissivity of the projector's white light lamp. The
inverse of the resistance is on the ordinate and the wavelength on the abscissa.
•
To establish such a curve, the 1
grating must be calibrated using the dispersion relation of diffraction gratings
working under normal incidence, that is:
With A as the wavelength, k the order (in this case 1), n the number of lines
140mm-1 and i the angle between order 0 and order 1 for the selected
wavelength (see figure 24). We can also use the helium-neon laser to calibrate.
•
Note: beyond 800 nm in the 1
radiation of wavelength greater than 400 nm.
Figure 25: Curve resulting from the combination of the spectral response of the
photoresistor and the emissivity of the source of white light.
st
order spectrum obtained with a diffraction
sin i = n.k.
st
order, there is superposition with the 2
wavelength
67
nd
order
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