Effect of Colorimetric Attributes on Perceived Blackness of Materials

While black is one of the most prevalent industrial colors in the world, the colorimetric attributes of what is considered black vary significantly and the range of subtle hue undertones can be numerous. However, no systematic study can be found in the literature pertaining to the potential role of colorimetric attributes in the perceptual assessment of blackness. We have experimentally determined that the perception of blackness is influenced by hue and chroma using psychophysical assessments of a range of black materials. In the initial part of this study a series of 2 × 2” precision cut glossy Munsell color samples comprising a hue circle with a lightness (L*) of approximately 20.5 and chroma (C*) between 4 and 6 were assessed using thirty color normal observers and a filtered tungsten daylight simulator (D65). Observers were asked to arrange samples in order from most like black to least like black with no time limits in three separate sittings. In the second part of the study 27 over-dyed woolen samples were arranged in 2”× 3” dimensions. Samples in this set had a lightness range of 14-16 and C* of 0.5-3.5, and were assessed by 25 observers in two sittings in the same manner. The third set of samples comprised 24 precision cut 2” × 2” dyed acrylic samples with a L* range of 10.5-12 arranged around the hue circle. Samples were selected such that they comprised three concentric hue circles of eight evenly spaced samples each. The samples were divided into five sets according to chroma: A (C* = 0.12-0.20), B (C* = 0.42-0.57), C (C* = 0.89-0.97), D (C* = 1.58-1.86), and E (C* = 3.343.46). For the assessment of samples in the third set 100 color normal observers were employed that repeated the assessments in three separate sittings with at least 24 hours gap between each sitting. Analysis of the data indicates that, irrespective of chroma, on average samples with hue angles between approximately 200° and 270° were perceived to be the most black, i.e., cyan to bluish-blacks. Blacks with hue angles above 315° or below 45° (reddish-blacks) were considered to be the least black. Chroma and lightness also influenced the perceived blackness but for the majority of samples the effect was less pronounced. Introduction The search of literature yields a very small number of manuscripts pertaining to the assessment of blackness. In 1980, W. D. Wright wrote a short article in Die Farbe on the perception of blackness in which he discussed the separation of television signals into a luminance channel and a chrominance channel [1]. To test whether this separation was valid, R.W.G. Hunt prepared a black-and-white slide and separated a chromatic slide of uniform luminance. Hunt and Wright found that combining the slides gave a reproduction of the original image, while the colors in the chromatic slide alone appeared garishly bright and unnatural. Wright discovered that the chromaticity was unchanged in the chromatic slide, and that the contrast provided by the luminance slide was critical to seeing the original image. Projecting the chromatic slide onto a uniform dark grey background did not return the colors to their original shades. From this, Wright drew a few conclusions. His first pertained to neural coding: current thinking at the time assumed that the black-white opponent channel was coded from the sum of the three types of cones in the retina. This idea led to the conclusion that blackness and luminance would be coded simultaneously, but his experiment showed that luminance must be separated from blackness at some point. His second conclusion was that the luminance alone was insufficient for color reproduction, as contrast is also important. Finally, he stated: “This raises the question of how we should measure or specify blackness. We might, perhaps, use the black-content scale of the Ostwald charts, or the black-white scale of the Natural Colour System, or the darkness-degree scale of the DIN System. What we do realize is that the blackness we are interested in is a subjective perception and not something that can be measured on any simple photometric scale. Moreover, the depth of the blackness that we perceive can be affected by quite small areas of contrasting lightness, for example, by the small highlights on the black grapes of Dr. Hunt’s first demonstration slide.” To further the understanding of perceptual blackness a formal study was initiated at North Carolina State University in 2004 to examine the role of colorimetric attributes on the degree of blackness perceived. Concurrently, the study aimed to assess color vision various models pertaining to the perception of blackness. Preliminary results of the psychophysical assessments were presented in a special Figure 1. Location of glossy Munsell, over-dyed wool and over-dyed acrylic samples used on the CIE a*b* plane. CGIV 2010 Final Program and Proceedings 83 meeting of the ISCC in Portland, Oregon, USA [2]. In a different study in 2006 the preliminary results of work towards the development of a blackness index were reported [3]. This paper reports some of the additional results of the on-going endeavor at North Carolina State University.