Introduction to illumination systems

In this lesson, an introduction to illumination systems is presented. Answers to questions like “What is illumination system design?” and “What are typical illumination systems?” are provided. 

Authored By Katsumoto Ikeda

Introduction

This article ntroduces a number of concepts that wil be expanded upon in the following articles. We will start from the initial question 'What is illumination system design?', before looking at what an illumination system comprises, example of some common systems, and an overview of the idea of imaging and non-imaging illumination systems.

What is illumination system design?

In one sentence, illumination system design is the design act to transfer the light from a light source into the desired distribution.

Light source

It follows that, for all illumination sytems, you must have a light source, and a method to characterise the output of the system.

Some examples of light sources are:

  • Light emitting diodes (LEDs)
  • Laser diodes (LDs)
  • Incandescent light bulbs, halogen lamps
  • The sun and other Black-body radiative sources like human beings and animals
  • Fluorescent lights
  • Luminescence sources
  • Point sources like the stars

For industrial applications, the most common light sources are from electric discharge such as halogen lamps, and electroluminescence, such as LEDs and lasers.

Some examples of the target distribution of an illumination system can be:

  • Flux:
    • Power or radiant flux (Watts or W)
    • Luminous flux (lumens or lm)
  • Flux per area:
    • Irradiance (W/m2)
    • Illuminance (lm/m2 or lux)
  • Flux per solid angle:
    • Radiant intensity (W/sr)
    • Luminous intensity (lm/sr or candella or cd)
  • Flux per area-solid angle
    • Radiance (W/m2·sr)
    • Luminance (lm/m2·sr or cd/m2 or nit)

Light sources, detectors, and measurement unit will all be covered in much more detail in subsequent articles in this learning path.

Along with hitting our target distribution for the illumination system, we want to transfer the light from the source as efficiently as possible. We can utilize common optical phenomena such as refraction, reflection, and even diffusion to change the direction of the light rays so that the target performance is met. Since the refraction, reflection, and diffusion can be controlled by optical surfaces, the intelligent use of optical surfaces become a crucial factor in successful illumination design.

Ray behavior

Examples of illumination systems

There are common uses for illumination systems, and we will list some examples, with a schematic image of use and brief explanation.

Laser diodes

Laser diodes (LDs) are used in many applications, and some of the more common illumination applications are barcode scanners, collimators, line generators, and projector systems. LDs are a very good light source for illumination, since most lasers have a very small illuminated size. This can be useful for collimation, and general control of the light rays. One thing to be careful of with laser diodes is the coherence of the light, which can lead to unwanted artifacts in the illuminated plane caused by diffraction, interference, and speckle phenomena.

Line generator

Lasers.JPG
By 彭嘉傑 - Own work, CC BY 2.5, Link

Lenslet arrays

Lenslet arrays are commonly used for uniformity of a distribution. The small aperture of each lenslet will have a small NA, and only account for a physically small portion of the entire illuminated area. A non-uniform light source can be sufficiently uniform within a small area. Another common use of lenslet arrays is to diffuse light beam, acting as a diffusion lens for angular illumination.

MLA

Backlight displays

Backlights are used for flat panel LCD displays, which do not illuminate on their own. Applications include cell phones and smartphones, up to large screen television displays, and sizes in between. Backlights comprise of different optical components, such as a light guiding panel, diffusing lenses, and brightness enhancing films (BEF). There are two types of backlight systems. The traditional edge illumination system uses an LED as a light source and the light diffuses along a light-guiding panel. Optical design is used to optimize the uniformity of the distribution.

LCD edge
(Reference article here)

The other method is direct lighting of the backlight display, which uses multiple LEDs facing the LCD display with a diffusing lens to spread the light. Since the display is illuminated directly, this method does not require the costly light guiding panel.

LCD direct

LED lighting

LEDs are a robust light source within the lighting industry. LEDs are small, solid and durable, with very good energy efficiency and longevity. Light up instantly, with abundant color options. Their advantages over traditional incandescent light bulbs, halogen lamps, and lasers make them a good choice for lighting under more strenuous conditions.

LED

Some use cases for LED lighting are automotive headlamps, road surface drawing lamps, LED light bulbs, and LED collimators.

Projector systems

A projector is a hybrid optical system in a sense, since it is an illumination system with an imaging component and possibly a nonimaging component in the system. Projection can be used in close proximity such as an intermediate illumination plane in an HUD system. Projection can also be used in far to mid range distances such as a projector display.

LCD display illuminators (the LCD display is backlit as part of the illumination)HUD

Lightguides

A light guide transfers light within an optical medium through multiple total internal reflections within the medium. The guiding of the light allows for bending the light almost in any direction, but requires understanding of the restrictions due to total internal reflection to avoid light leackage.

Lightguide

Differences between imaging and illumination

In the previous chapter, we have described the more common illumination systems we see today. Some of these are imaging systems, and some of these are non-inmaging illumination systems. Imaging optics are optical systems such as photographic objectives, that form an image of an object. A more complete discussion on imaging and sequential systems can be found here, in a separate learning path.

Nonimaging optics, as the name implies, does not use the concepts of objects or images as part of the design methodology. There is a fundamental difference between the two, and both imaging and non-imaging are useful concepts for illumination. 

How imaging can be used for illumination design

An imaging system with a real image can be a photographic objective or a projector system. If the image is directly projected onto a screen, we can use transfer the light from the object to the screen to achieve illumination. 

Double Gauss
Projection

For imaging projector systems, there are some parts that are different compared to a photographical objective, such as the telecentricity of the lens to achieve a more uniform distribution of light from the object. However, the philospphy of illumination using imaging is rooted by the simple raytracing that one can perform with a few rays. 

Although the above example is for a real object, there are also virtual images that are used in illumination. An imaging system with a virtual image can be eyepieces, finders, or optics for head-up displays. Although not directly related to illumination per se, serveral virtual imaging systems have an illumination component, and may need to be accounted for. For example, for a head-up display, the illuminated object (ususally an LCD) is projected to the eye via a virtual image. The amount of light and brightness that we see in the virtual image depends on how the light from the LCD image is transferred to the eye. Without careful thought of how the rays enter the eye, we may end up with a head-up display system that has a very low uniformity, or a head-up display system that changes brightness when we move our head around. 

There are several types of illumination system that use the theory and techniques of imaging systems in their design. An overview of some common examples follows.

Critical illumination

An image of the source to the illuminated surface or area can be a form of an illuminated system. Such a system is called critical illumination. Since the source is directly imaged to the illuminated surface, the uniformity (and also irregularity) of the source directly affects the uniformity of the resulting illumination. Therefore, the critical illumination is best used with a uniform light source. The projection lens also requires a moderate amount of space in between it and the image of the source for aberration controlled projection.

Critical illumination

The above example is a critical illumination system and shows the intermediate image of the source. The source is the object imaged onto an intermediate image with a condenser lens. The intermediate image is projected onto the screen with a projector lens. The marginal rays (solid red line) can be traced from the center of the source (the object) to the edge of the first pupil (roughly the condenser lens) to the center of the intermediate image. The marginal rays can then be traced from the center of the intermediate image to the edge of the projection lens, and then traced to the center of the illumination target (dashed red lines). The chief rays (solid blue line) can be traced from the edge of the source (the object) to the center of the first pupil (roughly the condenser lens) to the edge of the intermediate image. The chief rays (dashed blue line) can then be traced from the edge of the intermediate image to the center of the projection lens, and then traced to the edge of the illumination target, in this case the screen.

Notice how the concept of illuminating the screen is expressed in terms of imaging and ray tracing. With a well-performing condenser lens, a well-performing projection lens, and a relatively uniformly distributed source, critical illumination can perform very well.

Köhler illumination

Köhler illumination is a type of illumination system that is typically used for non-uniform sources, classically for filaments and lamps. The limitation of the critical illumination is the image on the intermediate plane, and any non-uniformity is imaged onto the final illuminated screen. Köhler illumination, on the other hand, uses a perfectly defocused image on the condenser and projection lenses.

Köhler illumination

In the image above, the source is imaged onto the projection lens with the condenser lens (solid blue and red lines). The projector lens images the condenser lens onto the illumination target (dashed blue and red lines). The marginal rays (solid red line) can be traced from the center of the source (the object) to the edge of the first pupil (roughly the condenser lens) to the center of the intermediate image, which in this case is the center of the projection lens. For the projection lens, the marginal rays (dashed red line) can be traced from the center of the condenser lens to the edge of the projection lens, and then traced to the center of the illumination target, in this case the screen. The chief rays (solid blue line) can be traced from the edge of the source (the object) to the center of the first pupil (roughly the condenser lens) to the edge of the intermediate image, which in this case is the edge of the projection lens. For the projection lens, the chief rays (dashed blue line) can be traced from the edge of the condenser lens to the center of the projection lens, and then traced to the edge of the illumination target. Notice that the marginal and chief rays are not connected, the solid red line and the dashed red line are not connected, and the slid blue line and the dashed blue line is not connected. That is to say that the marginal and chief rays are not connected between the first image (the condenser lens) to the second image (the projection lens). This makes the non-uniformity of the source an non-issue since the image of the source is not directly imaged onto the illumination surface.

Paraboloidal and ellipsoidal reflectors

The paraboliodal reflector is used to collimate the light from a light source, so another lens can be used to form an image onto a point.

Paraboloid reflector

The ellipsoidal reflector is an illuminating imaging method to collect light from a source onto a point. If we place the light source at one focal point of the reflector, the source is imaged to the second focal point.

Ellipsoidal reflector

In both diagrams we can see that only a subset of the light emitted by the source will hit the mirror and be directed or focused in the intended manner. Light emitted from right to left (not shown) would miss the mirror in both examples, and be wasted in the optical system.

Although the previous are examples of imaging concepts used for illumination, this is not the conventional method for illumination design. When non-sequential ray tracing was not available, concepts of imaging were the only method to theoretically achieve a uniform or arbitrary distribution. Although imaging systems can be used, there is not necessarily a need to use an imaging system for illumination. Nonimaging optics are often used. Either way, different thought processes, and different design tools (typically using non sequential raytracing) are needed for most illumination designs.

A primer on nonimaging optics

Nonimaging optics is a subset of optics that, compared to traditional imaging optics, do not attempt to form an image of an object. The primary objective of nonimaging optics is the transfer of light between the source and the illuminated target. That is to say that nonimaging systems and illumination optical systems try to acheive the optimal transfer of the light source to the desired distribution on the target of illumination.

Traditional optics have long been imaging optics, and designing them invloved tracing few rays to calculate first-order optics such as focal length, and third-order optics such as Seidel aberration theory. Although these methods involve intense mathematical calculations, they only require the tracing of a small subset of rays, following known paths, in order to be effective.

Nonimaging optics

Nonimaging optics has no bearing on the optical points A' and B', and there is more freedom to direct the rays the whereever we need them to be. In this aspect nonimaging optics has fewer constraints and rules as to how the rays need to pass through the optical system. This means that far more ray must be traced in order to accurately characterize a nonimaging system. As far as optical lens design, the non analytical nature of the target of optimization, such as "throughput", "optical efficiency", and "optical uniformity" can make the optimization process more abstract and less controllable. However, with level of computing power that is readily available today, it is feasible to trace millions, or even billions, of rays in a short time, making the design of nonimaging illumination systems much more robust.

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