The visual appearance of objects is given by the way in which they reflect and transmit

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

. The

color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...

of objects is determined by the parts of the

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

of (incident white) light that are reflected or transmitted without being absorbed. Additional appearance attributes are based on the directional distribution of reflected (

BRDF The bidirectional reflectance distribution function (BRDF), symbol f_(\omega_,\, \omega_), is a function of four real variables that defines how light from a source is reflected off an Opacity (optics), opaque surface. It is employed in the optic ...

) or transmitted light (BTDF) described by attributes like

glossy Gloss is an optical property which indicates how well a surface reflects light in a specular (mirror-like) direction. It is one of the important parameters that are used to describe the visual appearance of an object. Other categories of visual a ...

, shiny versus dull, matte, clear,

turbid Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wate ...

, distinct, etc. Since "visual appearance" is a general concept that includes also various other visual phenomena, such as color, visual texture, visual perception of shape, size, etc., the specific aspects related to how humans see different ''spatial'' distributions of light (absorbed, transmitted and reflected, either regularly or diffusely) have been given the name '' cesia''. It marks a difference (but also a relationship) with color, which could be defined as the sensation arising from different ''spectral'' compositions or distributions of light.

Appearance of reflective objects

The appearance of reflecting objects is determined by the way the surface reflects incident

. The reflective properties of the surface can be characterized by a closer look at the (micro)-

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

of that surface. Structures on the surface and the texture of the surface are determined by typical dimensions between some 10 mm and 0.1 mm (the detection limit of the human eye is at ~0.07 mm). Smaller structures and features of the surface cannot be directly detected by the unaided eye, but their effect becomes apparent in objects or images reflected in the surface. Structures at and below 0.1 mm reduce the '' distinctness of image'' (DOI), structures in the range of 0.01 mm induce ''

haze Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classificati ...

'' and even smaller structures affect the '' gloss'' of the surface. ::Definition
diffusion,

scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...

: process by which the spatial distribution of a beam of radiation is changed in many directions when it is deviated by a surface or by a medium, without change of frequency of its monochromatic components.

Basic types of light reflection

Appearance of transmissive objects

{, class="wikitable" , - ! colspan="2", Appearance of a Transmissive Object , - , , align="right", Figure 6: graduation of a ruler as seen through a translucent scattering layer (frosted glass). The original distinctness of image can be seen in the center of the lower graduation (around 0). On the left side the frosted glass is in contact with the ruler surface and it is 4 cm above the ruler surface at the right side of the image. With increasing distance between ruler graduation and scattering layer the blur (''haze'', also see

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wa ...

) increases and the '' distinctness of image'' (''clarity'') decreases. , -

Terminology

Reflective objects * ''Reflectance factor, R'' * ''Gloss reflectance factor, Rs'' * ''Gloss'' (at least six types of ''gloss'' may be observed depending upon the character of the surface and the spatial (directional) distribution of the reflected light.) * ''Specular gloss'' * '' Distinctness of image gloss'' * ''Sheen'' * ''Reflection haze, H'' (for a specified specular angle), the ratio of (

)

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...

reflected at a specified angle (or angles) from the

specular Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...

direction to the flux similarly reflected at the specular angle by a specified gloss standard. Transmissive objects ASTM Standards on Color & Appearance Measurement * ''

Transmittance Electromagnetic radiation can be affected in several ways by the medium in which it propagates. It can be Scattering, scattered, Absorption (electromagnetic radiation), absorbed, and Fresnel equations, reflected and refracted at discontinui ...

, T'' * ''Haze'' (

) * ''Clarity''

References

* R. S. Hunter, R. W. Harold: The Measurement of Appearance, 2nd Edition, Wiley-IEEE (1987) * CIE No 38-1977: Radiometric and photometric characteristics of materials and their measurement * CIE No 44-1979: Absolute methods for reflection measurements BRDF * F. E. Nicodemus, et al., Geometric Considerations and Nomenclature for Reflectance, U.S. Dept. of Commerce, NBS Monograph 160 (1977) * John C. Stover, Optical Scattering, Measurement and Analysis, SPIE Press (1995) Optics

Appearance of reflective objects

Basic types of light reflection

Appearance of transmissive objects

Terminology

See also

References