# Some Approaches for Light and Color on the Surface of Mars
## 4 Discussion  
Color science teaches [3] that, in a first approach, the perceived color of an object depends on three main factors: the spectral power distribution of the light source illuminating the object, the spectral reflectance/transmittance of the object, and the spectral sensitivities of photoreceptors in our retina. In the next paragraphs, we add some considerations about each one of these three factors in our current case. Given the interest of applied physics for the better understanding of Mars [6; 27; 28; 29], the current paper focused on lights on Mars' surface, considering their colorimetric characteristics (Sections 3.1.2-3.1.4), and proposed a model to compute Mars (sol-light) illuminants from an input CCT in the range of 2333 K-5868 K (Section 3.2.1). This model is analogous to the one recommended by CIE for daylight illuminants on Earth (clause 4.1.2 in [3]), but is based on 3139 theoretical spectral irradiances from the COMIMART model. Our model produced a very good match with the results generated by [PERSON] [7] using a limited number of 50 experimental irradiances from vehicles on Mars (Section 3.2.2). The spectra generated by our model display noise (mainly dips) that is present in the standard solar spectrum AM0 [30], while spectra published by [PERSON] appear as smooth curves (Figure 13). However, the similarity of results is encouraging. As expected, results in Table 2 show greater differences than those in Table 1, given that the model proposed in Section 3.2.1 was based on 3139 total spectral irradiances, not on the five illuminants proposed by [PERSON]. However, even in the case where the comparison between our model and [PERSON] results is poorest (M54; Figure 13), we found a high value of GFC (0.9977288) and a distance of 0.00322 \(\Delta E_{\text{u}\text{v}^{\prime}}\) units, which is lower than three Standard Deviations of Color Matching (3-SDCM), the usual acceptable chromaticity tolerance for light sources and observers with normal color vision [25]. This good match justifies further progress with our model. While D65 and D50 are the two currently CIE-recommended standard daylight illuminants [3], in the case of Mars, it seems appropriate to choose some sol-light illuminants with a lower CCT. For example, the average CCT corresponding to our 3139 total spectral irradiances is 3984 K (approximately, an M40 illuminant). This relatively low CCT value is attributable to the fact that we included high opacity values in our model. The average illuminant from Barnes has a CCT of around 5000 K (M50). Regarding objects analyzed in the current paper, we selected the 24 samples in the X-Rite ColorChecker(r) [15] because they are considered to be a small representative set of familiar objects found on Earth's surface. Obviously, rock and soil samples that are analogous to those on Mars observed from orbit [31] or in situ [32; 33; 34] are much more interesting and will be considered in future work. In relation to the sensitivity of the photoreceptors in the human retina, here, we followed current CIE recommendations [3] and adopted the CIE 1931 colorimetric observer for computations of CCT. However, the CIE 1964 colorimetric color observer was preferred to compute corresponding colors and color inconstancies of X-Rite ColorChecker(r) samples, because we assumed that most objects in our everyday life subtend viewing angles greater than 4\({}^{\circ}\). Currently, the CIE 1964 colorimetric color observer is assumed in most practical and industrial color applications, excluding the analysis of complex images. In the current paper, we used several tools of advanced colorimetry [20; 21], such as the computation of corresponding colors and color inconstancy indices. Such tools are necessary when comparing the colors of objects under different light sources, as emphasized in [16], whose