Performance targets of illumination design

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This lesson provides the fundamentals of illumination systems, in particular, the performance goals of illumination systems. This lesson is part of the Illumination Systems Fundamentals Learning Path. Questions like "What makes a good illumination design?" will be addressed. In this lesson, the various targets for the illumination system will be described so that you can clearly define the target parameters of your illumination design.

Authored By Katsumoto Ikeda

Introduction

This lessonprovides a discussion and examples off the performance goals of illumination systems. The question of "What makes a good illumination design?" will be unravled through the units and targets that are common in illumination design. 

What makes a good illumination design?

At the core of illumination design, there is only one goal for the performance:

"The optimal transfer of the light source to the detector"

However, we know that things aren't that simple. There is a multitude of methods to transfer the light. Some constraints make optimal transfer change depending on what we prioritize, such as size and performance. The detector can be any shape imaginable. Although common optical engineering properties such as color, cost, and ease of manufacture apply in this article, we will define the standard illumination requirements for various systems. With the lessons from this article, we will be able to define the critical parameters for our illumination system, and ensure that our illumination design is a good one.

The units of measure of an illumination system

Before diving into the core of the performance, let's define the parameters of our illumination system. There are two groups of the unit of measure, and subsets of measurement for each.

There are two aspects to the unit of measure of an illumination system: radiometric quantities and photometric quantities. While radiometry is the measurement of electromagnetic radiation, including the visible light spectrum, photometry measures the response of the human eye to light. The distinction between these two units is essential when we are considering our illumination system. For example, a laser diode with a wavelength of 905nm cannot be viewed by the human eye, and therefore, any photopic measurements always zero. On the other hand, it is vital to balance the energy of the light source with sub-UV blue light and near-IR red light as the human eye is most sensitive at about 550nm and will require more light on both sides of the spectrum of 550nm to achieve a balance for the human eye.

The terms may be confusing at first, but to summarize, the radiometric quantities are radiant flux Φ, irradiance E, radiant intensity I, and radiance L, while the photometric quantities are luminous flux Φ, illuminance E, luminous intensity I, and luminance L. The radiometric quantities can be further decomposed to spectral flux, spectral irradiance, spectral intensity, and spectral irradiance if the radiometric values are different in the spectral dimension of wavelength. In this case, it is good practice to call the radiant flux the total radiant flux to distinguish between the spectral dimension and the total spectral range.

*Please note that the designations of Φ, E, I, L are not universal, and sometimes P, H, J, and N are used for the corresponding radiometric quantities, while F, E, I, and B are used for the corresponding photometric quantities.

We want to be able to define these units correctly when we start the design, because a seemingly small error here will change everything. We don't want to start setting up our system for illuminance when the required target performance is the intensity. Different people have different definitions for what intensity (among other optical terms) means, so when we are talking to non-optical people, it is critical to make sure that the optical requirements are precise. If someone says, "I want the intensity of the surface to be 100W/m²", we need to be clear if they mean illuminance, or if they mean W/sr.

Below is a schematic image of the above photometric units. With a visual representation of the units, perhaps it is more intuitive to understand the different units of measure.

Source: PencilofRays.com

Key performance parameter: Uniformity

Uniformity is the extent of deviations in the distributed surface. Measures of uniformity are typically RMS and P-V, but sometimes it is described in terms of the slope or the change of the distribution in a given range.

Angular uniformity: uniformity over a solid angle

Angular uniformity is usually measured as intensity, and is the amount of flux per solid angle. Some examples of applications that require angular uniformity are:

• Time of flight (TOF) light source
• Solar concentrators

Surface uniformity: uniformity per unit area

Surface uniformity is usually measured as illuminance, and is the amount of flux per unit area.

Some examples that require surface uniformity are:

• Projector
• Backlight display illuminator
• In one dimension, line generators

Key performance parameter: Throughput and efficiency

Several factors can affect the amount of light passing through the system:

• The throughput or Étendue
• Absorption of the optical material
• Reflectivity of reflective surfaces
• Fresnel reflection between the optical surfaces

The loss of light that we can control with the design is the throughput. Depending on the shape and the amount of light that we can incorporate into the illumination system, the efficiency of the system changes. Fresnel reflection can change depending on the angle of incidence of the ray, and hence is slightly affected by the shape, but it is a non-dominant factor as a design parameter. This throughput can be referred to as efficiency. We measure the efficiency of the system by comparing the input light with the output light. In general, more throughput is beneficial, but not to the detriment of the system. There is rarely an exception to choose a solution with less throughput, but it comes down to a balance between:

• Performance, namely performance parameters other than throughput such as uniformity and color
• Size, which may be a constraint to begin with, and also determines the cost
• Complexity, which may harder to manufacture, harder to control the quality, which can cost more to make in volume
• Cost

Key performance parameter: Color

The perceived color of light is quantified by its chromaticity. Chromaticity is defined by the International Commission on Illumination or Commission internationale de l'éclairage (CIE), and the color matching functions give us the CIE 1931 chromaticity coordinates.

The chromaticity coordinates are represented graphically, and all possible colors of light are contained in this graph. The pure monochromatic spectral colors are denoted around the curved parameter of the graph. For most illumination systems, we use the central white region, which contains various shades of white. The curved line T_c(K) in the image above is the Planckian locus or blackbody locus, which is the path of color that a blackbody source as the temperature changes. A real-life example would be the change in color of heated metal, which changes from deep red, to orange, yellowish-white, white, and finally bluish-white at extremely high temperatures. The white light has a correlated color temperature (CCT) that is typically a target for the color. For example, a blackbody source like an incandescent lamp has a color temperature that very closely approximates a blackbody radiated source. Other light sources such as LEDs are classified similarly by color temperatures.

When designing a photopic illumination system, the target color "white" can be arbitrary. There are instances where the illumination design has the target color based on color temperature, and a target color based on CIE coordinates Cx and Cy. In practice, the target surface can have a requirement for color, such as CCT=4500K, or CIE(x,y) = (0.360,0.363). OpticStudio can export color accordingly. Radiometric applications are not perceived as color, but any possible wavelength changes that may affect the system should be taken into account.