3.1 Experimental setup
The setup described below, was used to perform the measurements of reection spectra. The setup consists of the laser source, optical system, sample holder, monochromator, photomultiplier, voltage amplier, radiometry voltmeter and a computer with the data acquisition software. Fig. 3.1 shows all these components.
Monochromator MS257 is the main part of the setup. It provides a wavelength resolution of 0.1 nm in the 170 1600 nm range. Monochromator has a quadruple grating with 1200 lines/mm and a motor driven slit assembly on the input and each of the two outputs. There are one input and two output ports. The default conguration of the monochromator supposes the output beam to pass out via the axial port. It is required to replace side exit mirror to adopt monochromator for the use with another output port. Quadruple-grating turret drive, ip mirror drive, slit assembly drive and integrated shutter are controlled automatically from the computer via the GPIB (IEEE-488) interface [28, 29].
Laser source illuminates the sample, the reected light gets through the diaphragm and the chopper to the monochromator input slit. The diaphragm is needed to re-move the light, reected from the unnecessary parts of the sample. The chopper, making 25 rotations per second, is needed to calibrate dynamically the zero input level it periodically hides the slit from the incident light and at these moments light level is considered to be zero and during the next cycle this value is subtracted from the received one.
The light of the desired wavelength on the output of the monochromator gets to
Figure 3.1: Experimental setup for reection spectra measurements.
the photomultiplier tube that is supplied with a high-voltage source. It is possible to vary the applied voltage in a wide range, but it is not desired, as the PMT amplication dependency on the voltage is unknown. Signal, produced by PMT, is amplied and fed to the radiometry system Merlin that, being also controlled by a computer via GPIB interface, processes the resulting signal and sends it to the PC. The intermediate amplier has a variable amplication coecient from 104 to 109. It is useful, as the Merlin system has a maximum input value of 5 V and it may be necessary to decrease the signal by a known factor in case of large signals.
The data acquisition software TracQ controls all the measurement characteristics and collects all tha data from Merlin system, allowing its preliminary processing.
The resulting data is stored on the disk in a simple and convenient format.
3.2 Light sources
For the experimental measurements it was decided to use not only laser source, but also a light of an incandescent lamp for the following purposes. The spectrum of
an incandescent lamp light is wide and smooth in the visible and infrared regions and has its maximum in the NIR. It can be approximately described by a Planck's law for the black body radiation
I(ν, T) = 2hν3 c2
1
exp(kThν)−1. (3.1)
Figure 3.2 shows an intensity distribution of energy through the wavelength for a typical lamp. That is why such a lamp could be used as a polychromatic source, although not so powerful on a single frequency as a laser. This information was not given above, in the theoretical part, as the author does not consider it to be important for an explanation of an operation and so on.
Figure 3.2: Spectrum of a typical incandescent lamp. [30]
As a laser source there was chosen a 10 mW He-Ne laser with the main wavelength of 632.8 nm. Harmonics peaks of 1.5x and 2x wavelengths: 949.2 nm and 1265.6 nm were utilized to get into the NIR region.
3.3 Working wavelengths
As the nal design of a device, using the proposed algorithm, is supposed to mea-sure the incoming light spectrum not integrally, but on few exact wavelengths, we have now also to use only some separated frequencies. Let us, this way, choose the amount of points that would be enough to cover NIR region more or less evenly.
It is necessary to take into the consideration the form of materials spectra given in part 2.4.2 and dyes spectra, given in part 2.5. It is good to remember that the increasing of data points amount will lead, in case of a device, to the increasing of the channels used, thus increasing the cost and lowering the reliability of the whole system. We may and should utilize the He-Ne laser, thus already providing ourselves with two working wavelengths 949.2 and 1265.6 nm.
Table 3: Probing wavelengths.
Thus, we provide a coverage of a NIR region with near 100 nm steps and do not get into local H2O absorption peaks (Fig. 2.1). It is proposed to set a window width for lamp-source points to be 10 nm from the following considerations:
• lamp source has no such localized power as a laser, so need to widen the window;
• there are serially produced lters for such windows [27];
• materials and dyes have no sharp spectrum areas in NIR;
• this is just an experiment, not a production.
For laser-source points it is proposed to choose the window, equal to the width of the corresponding laser peak. The exact values will be seen later from the preliminary measurements. We are not intended to use laser sources for all the measurements, only for method testing, as we do not have lasers for all the wavelengths and at the same time already have another sucient light source.
3.4 Samples selection
As written in part 2.5, it is proposed to choose from a variety of patterns only the camouages patterns of the armies, participating in the ISAF mission, as in this case it is possible to obtain extra information on pattern usage in reality, if necessary. Moreover, it is these armies that are usually pioneers in the introducing of a robotic warfare. This way, we have three types of patterns for investigation.
Table 4: Samples to be measured.
No Material Pattern/Colour
To measure dierent patterns it is proposed to x the structure of a fabric not to deal with dierent absorption for dierent materials. Vice versa, for measuring materials it is proposed to x colour, although it is not completely possible, as dyes
for each material will dier, as written in part 2.5. Table 4 shows, what samples are going to be measured. For patterned samples separate measurement of each colour of a pattern will be done in addition to standard all-sample-area measurement.
It is also proposed for this time not to utilize laser for full-pattern measurements, as we anyway have not got lasers for all the necessary wavelengths and use laser measurements only for the verication of the method proposed.
3.5 Filter modelling
For the current experimental purposes there is no necessity to have real lters, as there will be no real receivers and computing electronics. So we can only make a mathematical estimation of a lter behaviour. It is possible to simulate a lter by means of an equation, derived from the considerations of a bandpass lter op-eration, but it would be much convenient just to have a scalable piecewise linear function, representing a lter. Such a function or a dataset could be in our case acquired by digitizing the spectrum of a typical interference lter, shown on Fig. 2.7.
From [27] can be seen the average peak transmission coecients, Tx, for the desired lters with FWMH of 10 nm. This is given in Table 5.
Table 5: Filters peak transmission.
λ, nm 750 850 950 1050 1150 1265 1350
Tx, % 55 55 55 50 40 40 35
3.6 Simulating humidity
For the purpose of simulating humid atmosphere a simple water evaporating setup can be used. As shown in part 2.2.2, the amount of water vapour at xed relative humidity grows exponentially with the temperature, so we have just to boil the water with an approximately constant temperature to have a constant and humid enough atmosphere at the gap between the light source and the sample. My 1 kW kettle can serve the science a bit. The diameter of vapour-enriched area in this case is 11 cm.
Overheat air from the boiler will also introduce some geometric scattering. It seems to be more a feature, than a bug, because in natural conditions there can be found eects similar to road mirages [31].
3.7 Signal processing
In this work we are also going to simulate the signal processing in a real system, including normalization and sample determination. For the purpose of normal-ization it is proposed to perform additional measurement with a sample, having constant reection coecient, e.g. R= 10%and divide each spectrum (consisting, as agreed, of seven measured values) on the spectrum of that reference one.
Light intensity equation for the optical part with xed environmental conditions will be
Iλ =I0(λ)A(λ, l)R(λ)A(λ, l)T(λ) = I0(λ)A2(λ, l)R(λ)T(λ), (3.2) where Iλ is an incoming integral light intensity for the λ-window, R is a target reection coecient,T is a lter transmission coecient,Ais an atmosphere losses, I0 is a source integral light intensity for theλ-window. From (2.2) and taking into
consideration thatexp2(x) = exp(2x), we have
Iλ =I0(λ)A(λ,2l)R(λ)T(λ), (3.3) where l is a distance to the target. We may note also electronic part sensitivity and amplication S for each of the measurement channels
Ii =I0,iAi(2l)RiTiSi. (3.4) HereIλ is changed toIi to emphasize that it is already a digital signal of a channel i, ready for computer processing. Reference spectrum from 10% reective sample will look like
Ii,ref = 0.1I0,iAi(2l)TiSi. (3.5) Dividing (3.4) on (3.5) will give the plain values of the reectance
Ri = 0.1Ii
Ii,ref. (3.6)
The value of reference reectance (0.1) may not be taken into account, as the results are going to be compared between each other, having already been normal-ized. The formulas, derived above, may be later used for deeper investigations, like atmosphere dust eects, power calculations and so on.
After acquisition, the data from each channel is packed into a set. To compare these datasets between each other it is proposed to use Pearson's correlation coecient
rxy =
Pxiyi−xny p(P
x2i −nx2)(P
yi2−ny2), (3.7) wheren equals to the number of channels, i.e. seven. Then it is proposed to prove or reject a hypothesis of equivalence of dataset X and Y for various condence levels. Usually in such experiments the condence equals to 95%, but we will also check the hypothesis with a 99% precision. Using the Student's criterion
t= rxy√ n−2
p1−rxy2 = 2.236 rxy
p1−rxy2 . (3.8)
The criticalt-value for n= 7 and 95% is equal to 2.571 and for 99% it is 4.032 [32].
All the mathematical calculations can be done using MATLAB software.
3.8 The algorithm overview
Here is the step-by-step algorithm for carrying out the experiment described, in-cluding data acquisition and postprocessing.
1. Measure laser spectrum and verify that the are required peaks.
2. Find out windows widths for laser peaks.
3. Measure intensity 3 Iref from reference 10% reector.
4. Adjust amplication coecient and PMT voltage to make maximum value from previous measurement to be near 10 - 15% of the scale. It is preferred to maximize the PMT voltage and minimize the amplication coecient here to keep the possibility of a precise multiplication in case of weak signals.
5. Measure intensity Ipat from entire patterns.
6. Calculate normalized intensity for all the samples as I =Ipat/Iref. Let then these be Database samples.
7. Simulate high temperature and humidity by means of water vapour, increase distance between the setup and the sample, change angle on small random values.
8. Repeat measurements for the main types of samples. Let then these be Target samples.
9. Calculate correlation coecients.
10. Prove or reject hypotheses of equivalence of similar and non-equivalence of dierent Database and Target samples for 95% and 99% probabilities.
3 Measuring intensity means here and below in this part measuring intensity of a reected light at predened points, discussed in part 3.3, with a chosen window width.