J . -M . Deniel et al .: Radioprotection 2024 , 59 ( 3 ), 225 – 234 227
Fig . 2 . Principle of associating each pixel p in an image with a form factor ff p . In green : each p pixel in the picture corresponds to a rectangle in the focal plane of the camera . In purple : from the observer point of view ( i . e ., the camera optical center ), this rectangle corresponds to a solid angle v p making a θ p angle ( in blue ) with the viewing direction ( in red ). Then the form factor associated with pixel p is ff p ¼ 1 p v � pcos θ p .
Fig . 3 . INRS metal furnace with air temperature probe in the rear wall ( just above samples # 2 and # 5 ). Incandescent samples as described in Figures 5 and 6 : 1 = graphite melting pot , 2 = silica firebrick , 3 = GR 28 firebrick , 4 = ADC3W , 5 = SMV3O , 6 = LA234ESR , 7 = XC , 8 = E24 , 9 = LA2714 , 10 = GS400diam65 , 11 = CF 25 60 40 .
by ULS2048XL-EVO ( Avantes , 2020b )). It was calibrated in our laboratory and its stray light is corrected using the Barlier method ( Barlier-Salsi , 2014 ).
We measured [ 1000 ; 2500 nm ] spectral irradiance with an Avantes AvaSpec NIR256-2.5TEC InGas array spectroradiometer ( now discontinued and replaced by the EVO version ( Avantes , 2020a )).
2.2 Radiant sources
To characterize the spectral sensitivity of the red , green and blue channels of the Olympus E-M5 mkII camera used in this paper , we used a tunable monochromatic light source in the [ 380 ; 2500 nm ] range . It is composed of a halogen projector and the double grating OL750 monochromator ( Optronic laboratories , 2014 ).
A calibrated HGH RCN 1350 N1 blackbody ( HGH , 2020 ) serves as reference incandescent source . It operates up to 1350 ° C and emits energy following Plank ' s law through a 23 mm diameter opening .
A metal furnace at the INRS workshop was filled with samples ( see Fig . 3 ) whose spectral emissivity was measured at the LNE laboratory ( LNE , 2023 ). This furnace is not an item of laboratory but a real furnace used in industry . Its internal sides are made of silica firebricks and its opening is 40 cm wide and 23 cm high . The furnace air temperatures indicated in this paper were measured on its rear inside side . It can be set to about 1140 ° C , but we did not have any information about its probe .
The furnace was opened only during each radiometric measurements and picture captures for less than 30 s . This limited heat differences between the rear and front parts in the furnace , as well as equipment and operator burns . The operating mode was the following : the furnace was heated to its maximum for 24 h , to ensure the samples reached a homogeneous temperature of 1140 ° C . Then the furnace was cooled to 1100 ° C for an hour . The temperature was then reduced to 800 ° C by successive 1 h decrements of 100 ° C . Each time , the camera had to be removed then placed again in order to obtain a new picture . This prefigured what could be done in workshops and avoided destroying our equipment .
2.3 Camera
Our camera is an Olympus E-M5 mkII and its 12-40PRO optics were set at 25 mm ( equivalent to 50 mm 24 36 full format perspective ). Its 4640 3472pixel m43 sensor at ISO 200 allows capturing RAW picture data with 5405 useful levels per pixel channel ( 12.4 useful bits ( DxO , 2015 )). These levels are named counts and denoted by c in units .
To exploit picture capture precision , we converted the RAW data into lossless TIFF 16-bit pictures , while ensuring a linear response of the sensor when observing radiance , and no colour space conversion with DCRAW64 ( Dave Coffin , 1997 ) software and the following command : dcraw64 -v -4 -H 1 -W -r 1111-q3-T-o0-h . ORF Using our monochromatic light source and the AvaSpec2048-14 spectroradiometer , we determined the relative sensor sensitivity of the three E-M5 color channels . By “ relative ”, we mean that we determined the ratio between the three channels . In the following we show that an overall