Viewers of high dynamic range television (HDR, HDR-TV) expect a comfortable viewing experience with significantly brighter highlights and improved details of darker areas on a brighter display. However, extremely bright images on a HDR display are potentially undesirable and lead
to an uncomfortable viewing experience. To avoid the issues, we require specific production guidelines for subjective brightness to ensure brightness consistency between and within programs. To create such production guidelines, it is necessary to develop an objective metric for subjective
brightness in HDR-TVs. A previous study reports that the subjective brightness is proportional to the average of displayed pixel luminance levels. However, other parameters can affect the subjective brightness. Therefore, we conducted a subjective evaluation test by using specific test images
to identify the factors that affect the perceived overall brightness of HDR images. Our results indicated that positions and distributions of displayed pixel luminance levels on video affect brightness in addition to the average of displayed pixel luminance levels. The study is expected to
contribute to the development of an objective metric for subjective brightness.