8.5.1.4 Luminance

A perfectly diffuse Having hte properties of diffuse reflection or diffuse refraction. surface is special in that it exhibits constant luminance The area density of the luminous flux leaving a surface through a given solid angle. over all angles. All surfaces in Visual are modeled as perfectly diffuse Having hte properties of diffuse reflection or diffuse refraction., so this in practice results in the luminance The area density of the luminous flux leaving a surface through a given solid angle. of a surface being directly proportional to the exitance The area density of the luminous flux leaving a surface. of the surface and the illuminance The area density of the luminous flux incident on a surface. on the surface. The units of luminance The area density of the luminous flux leaving a surface through a given solid angle. are lumens/square foot/steradian or lumens/square meter/steradian.

Brightness is the perceptual analog of luminance The area density of the luminous flux leaving a surface through a given solid angle.. The mathematics and perception research are complex and beyond the scope of this manual. The simplest relationship is expressed by the Stevens Power Law shown at right. Effectively, a 2x increase in brightness requires 8x more luminance The area density of the luminous flux leaving a surface through a given solid angle..

Visual can report luminance The area density of the luminous flux leaving a surface through a given solid angle. by using Properties to change the Measurement Type and assign a Reflectance The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal, perfectly diffuse standard surface irradiated in exactly the same way as the sample. to a Calculation Zone after it is created. Select the Calculation Zone, click the Measurement list in the Calculation panel Subdivision of the Ribbonbar menu system that group related commands using vertical dividers. in the Properties tab A user interface object found in the Ribbonbar or Sidebar comprised of graphic and text buttons grouped based on function. and select "Luminance The area density of the luminous flux leaving a surface through a given solid angle.", then specify a Reflectance The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal, perfectly diffuse standard surface irradiated in exactly the same way as the sample.. Right-click or press Enter to apply the changes. Visual will immediately apply the Reflectance The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal, perfectly diffuse standard surface irradiated in exactly the same way as the sample. without recalculation.

Luminance The area density of the luminous flux leaving a surface through a given solid angle. can be displayed in the Rendered Display Mode By selecting from the Display Menu on the Calculations tab A user interface object found in the Ribbonbar or Sidebar comprised of graphic and text buttons grouped based on function.. (This is not the same as the Display button on the Home tab A user interface object found in the Ribbonbar or Sidebar comprised of graphic and text buttons grouped based on function..)

Brightness can similarly be displayed by making that choice form the sub-menu.