Clarity is evaluated at angles of less than 2.5 degrees. This causes a reduction in contrast of objects viewed through the film. Low clarity films have a distorted unsharp appearance.


A collimated light source is one that produces rays that are nearly parallel, and have a near-zero beam spread. A common source of collimated light is a laser. By using lenses, mirrors, and back reflectors, one can design a collimated beam from nearly any light source.

Color Rendering Index (CRI)

Color Rendering Index (CRI) is a quantitative measure of color fidelity of a light source of <5000K when compared with an ideal light source (typically a black body radiator.) Generally, higher CRI will produce a more pleasing light. Must be noted with Color Temperature.

Color Temperature

Correlated Color Temperature Characteristic of visible light is determined by comparing its chromaticity with that of a black-body radiator (like the sun or some incandescent light bulbs.) Higher temperatures indicate ‘cooler’ bluer light. Lower temperatures indicate ‘warmer’ redder light. For reference, daylight is 5000K while incandescent light bulbs are around 3000K.


Diffusion in photonics refers to a situation where photons travel through a material and their behavior is dominated by scattering. Fusion Optix offers diffusion films and lenses which act as a negative lens increasing the angular output of light from a source.

Diffusion Angle

Diffusion angle (FWHM) is defined by the exit angle of light through a material when a collimated light source is used. It is defined in two directions. When both directional angles are the same a circular exit pattern is formed; when the directional angles are different an elliptical pattern is formed.

Diffuse Reflection

Diffuse reflection is produced from an uneven surface, seemly reflected at a number of angles.


Diffusion in photonics refers to a situation where photons travel through a material and their behavior is dominated by scattering. Fusion Optix offers diffusion films and lenses which act as a negative lens increasing the angular output of light from a source.


Efficacy is the capacity to produce light. Measured in lumens per Watt (lm/W).


Efficiency is the ratio of output divided by input; in optical terms, it is the ratio of the amount of output light / amount of generated light.

Final Output Angle

The final output angle depends on the light source and its angular profile along with the diffusion film’s angular profile. For most light sources the final output angle can be approximated by √(LED viewing angle)² + (Film Diffusion Angle)². When using a collimated light source the output profile of a Fusion Optix Diffusion film is near lambertian.


Glare is a term used in the lighting industry to describe over-illumination, or the presence of light intensity beyond that required for a specific activity. Often refers to large amounts of off-axis light.

Gloss vs. Reflection

Gloss is a surface characteristic that describes the interaction of light and the surface of a film. Gloss is affected by refractive index, angle of incidence and surface topography. Reflection describes the percentage of light that bounces off the surface.


Describes the percentage of light that deviates from the incident beam more than +/-2.5 degrees off axis on average. High haze films have a milky appearance.

Index of Refraction

The Index of Refraction (or Refractive Index) is the property of a medium (air, glass, PMMA, PC, etc.) which describes the reduction of the speed of light inside that medium. Noted as “n” in Snell’s law.

Insertion Loss vs. Transmission

Transmission is a percentage of light that passes through the film. It is measured using a semi-collimated light source. Insertion loss is measured using a non-collimated light source and describes the amount of light that is not able to pass through a film.

Lambertian Surface

A Lambertian surface is equally bright when viewed from any angle with the human eye.

Luminance vs. Illumination

When designing lighting fixtures it is imperative for the designer to understand the difference between luminance and illumination. Luminance is defined as the amount of light that passes through or is emitted from a particular area and falls within a given solid angle. Luminance is measured in cd/m2, or lm/sr/m2. Illuminance is the total luminous flux incident upon a surface per unit area. It is measured in lux, or lm/m2. Illumination does not depend on the angle from which the light came. This is particularly important when balancing lux measurements on a surface when increasing the angular component of a light source when adding a diffusion angle.

Prism Pitch

Distance between two prism structure peaks, which affects the light control properties of the film. Typically smaller pitches are used for smaller displays.


Refraction is the change in ray direction due to change in speed when a ray passes from one medium to another.

Snell’s Law

Snell’s Law describes refraction; relationship between angles of incidence and speed as light travels through a medium.

Specular Reflection

Specular reflection is perfect mirror like reflection as described by the law of reflection which occurs with a very even flat surface.

Total Internal Reflection (TIR)

Total Internal Reflection (TIR) occurs at the boundary between two refractive indicies. If the refractive index is lower on the other side of the boundary, and a ray strikes that boundary at an angle larger than the critical angle (WRT the normal to the surface), no light can pass through the boundary and effectively all light is reflected. The critical angle is the angle of incidence above which total internal reflectance occurs.

Viewing Angle of LED

The full width at half maximum of the angular output profile of an LED. Typical values range from 90-160°. Most LEDs are at 120°.