EXPERIMENT - IV
SPECTRAL TRANSMITTANCE OF COLOR FILTERS
NOTE: On arriving to the lab, the lamp of the monochromator must be switched-on immediately, so that it has enough
time to warm up and stabilize.
1- OBJECTIVE AND THEORETICAL BASIS
The objective of this experiment is to obtain the spectral transmission curves of some color
filters. The actual measurement is performed over a set of wavelengths along the visible spectrum.
The fundamental tool for this experiment is the monochromator (see Fig.1). This instrument
contains a polychromatic light source (a bulb lamp), and a diffraction grating as the dispersion
element. By rotating this grating, the output wavelength through a slit window is changed, therefore
obtaining a monochromatic light of the desired wavelength. For performing each of these
measurements, the outgoing intensity must be measured with a photodetector system, either directly
or interposing the filter under study. The ratio between these two values is the transmittance.
1
6
2
4
2
3
5
7
8
10
9
11
12
Fig. 1 Scheme of the measuring system. (Points 1 to 7 are
inside the monochromator instrument)
1.- Light source
2.- Lenses
3.- Mirror
4.- Entrance slit
5.- Reflection diffracton grating. (BASIC DISPERSION
COMPONENT OF THE MONOCHROMATOR)
6.- Wavelength adjustment wheel
7.- Output slit
8.- Color filter
9.-Photodetector
10.- Power source (V-constant) for the detection circuit
11.- Voltmeter
12.- Controlling computer.
2- MATERIALS
• Monochromator (points 1 to 7 in Fig. 1)
• Detection system (photodetector (9), voltage source (10) and voltmeter (11).
• Data acquisition system (if implemented).
• Color filters (8).
3- OPERATING METHOD
1.- Remove the detector and turn the -wheel (6) along the visible spectrum while looking at
the output slit. Check the wavelength at some characteristic color and the output color at some
wavelength of your knowledge.
2.- Mount and check the photodetector piece (9).
3.- We now set the wheel at 450nm, as the starting point of a scanning process through the
visible range. We shall measure the output signal Vo(voltmeter) from 450 to 750 nm in 20nm steps,
with no filter in the holder. This set of measurements will represent the light incident on the filter.
For a good measurement, it is advised to wait a few seconds between two consecutive
measurements, in order to let the signal stabilize. However, it is preferable to keep constant this
interval between signals, and avoid longer periods, not only for the excess in the overall time of the
scan, but also because of the drift suffered by the signal produced by the system (heating of the
lamp and photoresistance).
[If the acquisition system is implemented, the reading program must be run. It will read the
voltmeter and produce an average value].
4.- Put a color filter of your choice (filter #1) in the filter holder, and repeat the measurement
process for the same set of wavelengths, measuring V1
5.- Repeat step 4 for another filter (filter #2)
6.- Repeat step 4, this time putting both filters together (filters #1 and #2 in the holder).
7.- With the results found for the individual filters, calculate the spectral transmittance
curves for each . (For filter i: abscisae-; ordinate= Ti= Vi/Vo)
8.- Represent the curves Ti vs  in a figure.
9.- Calculate the transmittance T12 corresponding to both filters put together. Represent the
result in a plot T12 vs . Together with this plot, include another, corresponding to the product T1·T2
of the transmittances obtained by putting the filters separately. Compare the curves that, ideally,
should be equal.
4- QUESTIONS
1.- Comment the connection among the colored aspect of a filter and its spectral transmittance.
2.- What conclusions do you reach from the comparison of T1·T2 and T12?
3.- Do you think that the linearity of the system is guaranteed?
NOTE 1: DETECTION SYSTEM
The detector is a photo-resistance (a solid
Ph
state element), denoted Ph in Fig.2. On the arrival
of a photon, it produces an electron-hole pair.
10 K
Vs
This resistance is in a constant-voltage circuit, fed
by the voltage source of V volts. The charges
+
created by the photon originates an electric
V
current, that is, in principle, proportional to the
(*)
light intensity. On increasing the light and the
number of electron-hole pairs, the conductivity of
this element increases (the resistance, i.e. the
inverse of the conductivity, will increase
accordingly). By introducing a second resistance
Fig. 2 Scheme of the detection system. (Points 9
(10K in our case) in serial connection with Ph,
to 12 in the scheme of Fig 1)
the voltage will fall on passing through it to an
(* ) Only if connected to a data acquisition
amount of Vs, as read in the voltmeter. Then VPh+
system.
Vs = V (constant).
By following through all this reasoning, we obtain that the value Vs, as measured with a voltmeter
between the ends of the 10K resistance, increases when the light incident on the Ph increases.
NOTE 2: PHOTO-RESISTANCE
The generation of electron-hole pairs in any semiconductor device
is strongly dependent on the frequency of the radiation received, and also
on the forbidden energy gap between the valence and conduction bands.
For instance, for an intrinsic Si semiconductor, the efficiency is very low
in the blue side of the spectrum, and increases through the visible and up
to 900 nm, where it suddenly drops. A material typically used for these
elements is the Cadmium Sulphur CdS.
Fig. 3 Image of a photo-resistance.