## Understanding the Photometric Light Measurement Units

There are two sets of light intensity measurement units:
The Photometric units measure the intensity of visible light** as it is perceived by the human eye, and the Radiometric units measure the intensity of electromagnetic radiation***, which is the broader physical phenomenon of light, including the whole spectrum of radiation beyond visible light** (like x-rays and infrared radiation for example).

Light intensity is generally measured in three ways:

1. The directional intensity received from a light source as it is measured from a point in space. i.e Luminance in Photometric units or Radiance in Radiometric units.

2. The total light intensity output a light source emits to all directions i.e Luminous Flux in Photometric units or Radiant Flux in Radiometric units.

3. The amount of light intensity received by a surface from all directions i.e Illuminance in Photometric units or Irradiance in Radiometric units.

Similarly to the way measurement of kinetic power is based on the power of an ideal horse, the Photometric measurement units base the scale of light intensity on the light emitted by an ideal candle.

Luminance (Candela):
When measured from any point in space, the Luminance of an ideal candle seen from that point is measured as 1Candela‘ i.e. 1CD‘.
> In 3D rendering, a photometric IES file describes a light source’s light beam pattern by listing the Luminance or CD of the light source in different directions.

For light emitting surfaces like LCD screens Luminance is measured as Candelas per 1 square meter of surface i.e. CD/m2. Typical LCD computer monitors for instance, have a Luminance of about 250 CD/m2. imagine your computer screen displaying pure white and extended to an area of 1m x 1m, the light intensity perceived from it would be as if about 250 candles were spread on the whole area.

Luminous Flux (Lumen):
The amount of light emitted by an ideal candle through 1 solid angleSteradian‘**** conic beam distribution is measured as 1Lumen‘ or 1lm‘. the total Luminous Flux of the candle in all directions is 4 x PI lumens i.e 12.56 lumens which is simply the whole surface area of the unit sphere.
> The total amount of light produced by different kind of light bulbs is usually specified by Luminous Flux measurement i.e how many Lumens does the light source output.
> If sn optical reflector is placed next to a light source, focusing all it’s light output to a narrow direction, it wont change the light source’s Luminous Flux (Lumen) output, but since the same Luminous Flux will be focused to a narrower beam, it will have a higher Luminance (CD) measured from that direction, and therefore surfaces at that direction will be receive a brighter Illumination (Lux) (see below).

Illuminance (Lux):
A 1 m2 (meter squared) area surface, receiving illumination of 1 lumen has a measured Illuminance of 1 lux or 1 lx. Illuminance is measured by how many lumens a surface receives per square meter.
> In photography, the amount of Illuminance at which a surface is lit is important for determining the proper photographic exposure for the picture.

The inverse-square law:
As a light beam travels through space it’s distribution covers a larger and larger area, therefore it’s energy per area is reduced. the light energy a candle emits through 1 solid angle steradian, in a distance of 1 meter will cover an area of 1 meter squared, therefore the area of 1 meter squared will receive 1 lumen of light energy and will be illuminated with an illuminance of 1 lux. as that 1 lumen of light energy travels another 1 meter further, to a distance of 2 meters from the candle, it spreads and covers an area of 4 meter squared. each square meter of the 4 now receives just 1/4 of a lumen, so it’s illuminated by only 1/4 lux. as that 1 lumen of light energy travels another 1 meter further, to a distance of 3 meters from the candle, it spreads and covers an area of 9 meter squared. each square meter of the 9 now receives just 1/9 of a lumen, so it’s illuminated by only 1/9 lux. after a distance of 4 meters, the same 1 lumen on light energy will be spread on an area of 16 meter squared, so each square meter will be illuminated by 1/16 lux. you can already see the emerging pattern, the illumination intensity is inversely proportional to the square of the distance to the light source. This phenomenon is referred to as ‘The inverse-square law‘, and in practical terms it means that surface illumination is greatly influenced by it’s distance from the light source.

Notes:

* Radiometric units measure light intensity using Watt light energy units.
note that this isn’t the Watt measurement units of electric consumption we’re used to for classifying electric light sources with, but a Watt measurement of the actual energy in the light itself.

** Electromagnetic radiation of wave lengths that stimulate the human eye.

*** Also referred to as ‘light‘ in physics.

**** A ‘Steradian‘, also referred to as ‘square radian’ is a measurement unit of 3D conic span or ‘solid angle’. a solid angle of 1 Steradian beginning at the center of a unit sphere covers exactly an area of 1 squared on the surface of the sphere. (the whole surface area of the sphere being 4 PI). The Steradian can be thought of as the Radian’s 3 dimensional ‘cousin’.

Related posts:
IES lighting in CG
Fresnel reflections

## Understanding Fresnel Reflections

What we refer to in CG by the term “Fresnel Reflections” or “Fresnel Effect”, is the way Specular Reflection intensity changes according to light \ surface incident angle, and it is a basic optical property surfaces.

Specular reflection intensity changes according to light incident angle, and behaves almost like a perfect mirror at grazing view angle.
The reason we call this natural reflection behavior “Fresnel Effect” or “Fresnel Reflection” is that the equations describing the how reflection intensity changes according to incident angle were invented by the French Physicist Augustin-Jean Fresnel, and in early CG days, not all systems knew how to calculate natural reflections or reflections at all for that matter. So in CG we ended up treating this as something special, when in fact it’s not special in nature, it was just special in the early days of ray-tracing.

When rendering Fresnel Reflections, the reflection intensity isn’t determined by a linear blending percent like mixing a layer.
It’s determined by a factor called “Refractive Index” or “Index Of Refraction” i.e. IOR.
The IOR value is derived from the physical material’s density, which is the key factor determining both reflection intensity and refraction amount.

Examples of some physical IOR values*:
Air (vacuum): 1.0
Water: 1.33
Glass: 1.52
Diamond: 2.417
* Physical values differ between different measurements and samples of materials so you might see differences between different data sources.

This ball is rendered without “Fresnel Reflections”.
Its Specular reflection is blended consistently at 50% over the diffuse color (reflection), not affected by the light/view incident angle.
The result looks wrong for a natural material. It may look like a dielectric material (non metal) that’s coated with a silvery coating, but it can’t look correctly like glossy plastic or glass.

This ball is rendered with “Fresnel Reflections”.
The reflections look natural for a dielectric material (non metal), because they are dim at perpendicular incident angle and intense at grazing view angle, hence seen mostly at the sides of the ball accentuating its contour.

Theoretically Specular Reflection for all types of materials should be calculated using what we refer to in CG by the term “Complex Fresnel”, that is reflection equations that take into account both the Refractive Index (IOR) and Extinction Coefficient for 3 primary colors (spectrum wave lengths).
*Complex Fresnel component values for different materials can be found on https://refractiveindex.info/.
In practice, for Dielectric materials (non metals), most common production rendering systems use what we refer to in CG by the term “Simple Fresnel” or “Simple IOR”, that is calculating the reflection for all 3 primary colors using a single Refractive Index value, which is the surface’s Refractive Index for the Green primary color.
This method has proven itself to be very efficient for rendering non-metallic surfaces (dielectric materials).
Rendering metallic reflection using Complex IOR produces the most realistic color and reflection* for metals.
*In metallic surfaces the color is the reflection color itself and not a separate Diffuse component.
Some rendering systems like Arnold 5 for example have implemented a general form* of Complex IOR into their physical surface shader, Complex IOR reflection can also be rendered via OSL shaders that can be found on the web (or written..).
*I’m using the term ‘general form’ because these implementations don’t include input for Complex IOR values but just a general metallic reflection curve, that interpolates manual color selection.

This golden monkey statue (“Suzanne”) is rendered in Cycles using a Complex Fresnel procedural node:

Popular useful cheats for mimicking metallic reflection without complex IOR are to set a very high (non physical) simple IOR value, like 15 to 30 which forces the Fresnel reflection to become more metal-like, or turn Fresnel reflection completely off, turning the specular reflection into a perfect mirror reflection, or create a custom reflection/angle curve/ramp that produces the effect of the metallic reflection color and intensity changing by incident angle, see example here.

In many popular production renderers, the physical surface shader uses a single IOR parameter. Some rendering systems allow using 2 different IOR parameters, one for calculating reflections and the other for calculating Refraction.
* physically correct dielectric materials should be defined with the same IOR value for both reflections and refraction. using different IOR values for reflection and refraction allows useful cheats like creating transparent a material that is modeled without any thickness or defining a transparent glass that has silver reflective coating like sunglasses sometimes have.

Notes:

1. IOR lists on the web, that display only simple IOR values like this list, are not valid for metals, and produce wrong results.
*Using simple IOR values for dielectric materials however is very efficient.
2. There are parts in the CG industry where in daily slang language, the term “Fresnel” is used to refer to any shading effect that is view-angle dependent,
Usually referring to the shading properties appearing at the “sides” or contours of the model.
3. There are some CG systems that use the term Fresnel to refer to a simple linear or non-linear incident angle blending effect, that should actually be called “Facing ratio” or “Perpendicular-Parallel” blending (falloff).
This is wrong because IOR based Fresnel reflection intensity produces a specific physical Reflection intensity/view angle function curve, and not just a linear or simple power function.
See example in UE4’s Fresnel node.
4. Some PBR rendering systems using implementations of the Disney principled BRDF don’t use a Fresnel IOR input value for dielectric materials, but instead use a simpler 0 – 1 (or higher) Specular input parameter that produces dielectric reflections of approximately 1.0 to ~1.8 IOR value range, which generally covers the range between no reflection at all to common gemstones.
This approach may be inconvenient for rendering artists that are used to setting IOR values, but it has a significant advantage of allowing the usage of an LDR 0 – 1 range (regular image file) texture map for the Specular input, and that way be able to define different material reflections on the same object, rather than having to create a physical IOR map that has encode higher than 1 float values or be mapped to that range at the shading graph.
* See Blender’s Pricipled BSDF and Unreal Engine’s PBR Material.
* Blender’s Principled BSDF actually allows setting Specular values higher than 1 to render material denser than IOR 1.8 (like a diamond for example).
5. Many modern production renderers use Schlick’s Approximation to render Fresnel reflections, a simplified Fresnel formula that is both faster to compute and better suited for microfacet glossy reflection models.

Related:

## IES Lighting in CG

IES stands for Illuminating Engineering Society, it is the organization responsible for creating and maintaining industrial standards for design and manufacturing of artificial light sources.

In 3D rendering, an IES file or “photo-metric file” is a text file containing a photometric description of a light source’s beam spread , pattern and intensity, allowing for faithful depiction of the light source in 3D renders.
Most modern 3D rendering software support IES lights, that is allow loading IES files into the software and lighting the 3D scene using the light source described in the IES file.

Lighting manufacturers make measurements of their light fixture model’s physical light output and create IES files available for download on their websites.
This allows architects, lighting designers, and interior designers to download the files and realistically visualize the light sources effect on their projects.

CG artists use IES lights to add realistic spotlight beam patterns to their renderings and animations, such that can’t be created using regular simple 3D light sources.

Examples of IES lights rendered with V-Ray for 3ds max:

## IPR – Interactive Production Rendering

IPR is an abbreviation of Interactive Production Rendering,
A rendering mode that continuously updates the rendered image as changes to the scene take place.

## Sprite Sheet – Storing animation frames in one texture

An efficient way to store 2D animation for games is to layout all the frames withing one texture called a ‘Sprite Sheet’ or ‘Texture Atlas’.
This saves resources by avoiding multiple texture loading operations and only animating the UVs of the shader to display the needed image at each frame.
Sprite Sheets are also used to pack various states of game graphics and textures for multiple objects in one file.

Creating Sprite-Sheets:
Sprite Sheets can be created manually using any image editing software,
For an automated process and more control, a specialized software like Texture Packer can be used.
And it can also be done automatically in Adobe Animate (Flash).
* There are many more solutions / scripts that will do that you can find on the web…