Radioprotection No 59-3 | Page 89

232 J . -M . Deniel et al .: Radioprotection 2024 , 59 ( 3 ), 225 – 234

Table 2 . Estimated furnace and samples temperatures , in ° C , at areas marked in Figure 11 ( 2 ). NB : at 1140 ° C , the coil ( d ) pixels red channel is saturated .

Probe

Estimated temp . ° C temp . Mean Min Max

matrix , that we wanted conservative in terms of prevention from occupational exposure .

4 Discussion

4.1 Temperature consistency between sources and our approach

We first tested our method against a blackbody . In this experiment , the source was calibrated and we could verify that its radiance followed Planck ' s law precisely . Temperature indications could be considered close to reality at ± 2 ° C and emissivity was 1 in the 1000 to 2500 nm wavelength range .

As shown in Table 1 and illustrated in Figure 9 , our method is able to estimate the temperature of a blackbody with a precision of þ2 % to þ 7 %. In our opinion , this is conservative in the sense that exposure to IR will not be under estimated .

On the contrary , our metal furnace was not calibrated and thedifferencebetweentheairtemperatureattheprobeand that of the samples was unknown . That is why the comparison in Table 2 should be considered with caution . Nevertheless , the estimated temperatures except ( d ) at 1140 ° C differed from probe indication by -5 % to þ 10 %, which is realistic .

The estimated temperature pictures in Figure 11 show dark areas . Given the camera exposure time , they correspond to surfaces that do not emit enough light to be considered , whereas their IR emission should not be neglected . To overcome this limitation , picture analysis should rely on bracketing ( Debevec et al ., 2023 ) that consists in several successive pictures with different exposure times , exactly like in videophotometers ( Safdar et al ., 2016 ). This way , each pixel color should be chosen in the picture where the color channel levels are most exploitable with two-fold benefits : accounting for all the incandescent materials and improving temperature estimation precision .

4.2 Emissivity : limits and dependency on temperature

As it was explained before , our first remark about emissivity is that the picture analysis considers almost only graphite as the incandescent material : in parallel with ðg = r ; b = rÞ to T matrix , the method also keeps in memory the corresponding ðg = r ; b = rÞ to material index m matrix . Its non-empty cells almost always refer to graphite .

The first reason appears in Figure 5 and 6 : this material has the highest emissivity , close to a 85 % grey body . In this case , graphite emits the most at a given temperature . This is why the matrices of the method are filled preferentially with graphite .

A second and potential reason could be called “ cavity effect ”: in the furnace , light is first emitted because of incandescence , then reflected multiple times . We suspect