A Comprehensive Guide to Light Distribution Curve

The luminous intensity of a light fixture in different directions in space is usually not the same. We can use a distributed photometer to record the data of the light intensity distribution of the light fixture in space, and then import the data into the coordinate system to get the light intensity distribution curve, which is what we usually call the light distribution curve.

What is Light Distribution Curve?

The light distribution curve is actually a representation of the distribution of light emitted by a lamp or light source in space. It can record the light intensity of the lamp in various directions, as well as parameters such as the lamp’s luminous flux, number of light sources, power, power factor, size, efficiency, manufacturer, model, etc.

There are several types of photometric curves, including polar and Cartesian curves, as well as various kinds of light-intensity graphs.

Polar Distribution Curve

A polar distribution curve represents the distribution of light intensity in the photometric plane through the center of the light source. The curve is created by plotting the light intensity at different angles from a specific direction and connecting the vector endpoints. If the light fixture has a rotational symmetry axis, the intensity distribution on one photometric plane through the axis can illustrate its spatial intensity distribution.

For light fixtures with asymmetrical light distribution, multiple photometric planes intensity distribution curves are required to demonstrate the spatial intensity distribution.

Cartesian Distribution Curve

Cartesian distribution curves are used for spotlight fixtures, where the light beam is concentrated in a very small solid angle. The vertical axis represents the light intensity (I), while the horizontal axis represents the beam angle.

If the light fixture has a rotational symmetry axis, a single photometric curve is sufficient to represent it. For asymmetric fixtures, multiple photometric curves are needed.

Equal Light Intensity Curves

Equal light intensity curves connect the endpoints of vectors with the same light intensity. A graph composed of a series of these curves, arranged in proportion to their intensity, is called an equal-intensity graph. Common formats include circular grid graphs, rectangular grid graphs, and normal arc grid graphs.

Currently, rectangular grid graphs are commonly used for spotlight fixtures due to their ability to represent both the intensity distribution of the light fixture and the area distribution of the light.

Classification of L Light Distribution Curve

International classifications categorize lighting fixtures based on their upward and downward light flux. There are five primary types of lighting, namely direct lighting (downward light flux accounts for 90-100%), semi-direct lighting (downward light flux accounts for 60-90%), general diffused lighting (downward light flux accounts for 40-60%), semi-indirect lighting (downward light flux accounts for 10-40%, upward light flux accounts for 60-90%), and indirect lighting (downward flux only accounts for 0-10%, upward flux accounts for 90-100%).

Direct Lighting Fixtures:

These fixtures are highly efficient, using most of their light flux (90-100%) to illuminate the work surface directly. They minimize light loss, typically achieving an efficiency of over 80%. However, the more light directly projected from a limited area, the more significant the shadow and glare, possibly creating strongly reflected glare.

Example fixtures:

Conical enamel shade, block mirror shade, square grille fluorescent lamp, prismatic light plate fluorescent lamp, downlight (ordinary bulb), downlight (reflective lamp), mirror reflector, one-way grille fluorescent lamp, spot lamp (mounted on a rail)

Semi-Direct Lighting Fixtures:

These fixtures direct 60-90% of their light flux downwards and 10-40% upwards. The downward light increases efficiency, while the upward light increases ceiling diffused light, reducing the brightness contrast between the ceiling and the fixture.

General Diffused Lighting Fixtures:

These fixtures direct an approximately equal amount of light upwards and downwards, each accounting for about 40-60% of the total light flux. This balance provides a good brightness distribution throughout the room and reduces glare.

Semi-Indirect Lighting Fixtures:

These fixtures direct 60-90% of their light flux upwards and only 10-40% downwards, illuminating the ceiling primarily and creating a bright and spacious feeling. However, the indoor illuminance might not be high enough due to this.

Indirect Lighting Fixtures:

These fixtures direct 90-100% of their light flux upwards and less than 10% downwards. The ceiling and upper half of the walls are bright, creating an autumn-like, clear and open feeling.

No single type of lighting fixture can be exclusive; each type has characteristics that may or may not meet the requirements of a specific application. To appropriately evaluate a lighting fixture, consider whether it meets the basic visual requirements and creates a comfortable visual environment.

Axial Symmetry in Light Distribution

From the perspective of symmetry, light distribution curves can be categorized into three types: Axial symmetry, Symmetry, and Non-symmetry.

Axial Symmetry

In axially symmetric light distribution, the distribution of light is symmetric about a specific axis. This means that if you rotate the lighting fixture around this axis, the light distribution pattern will remain unchanged. Axially symmetric light distribution is commonly seen in circular lighting fixtures such as pendant lights or recessed lights, where the axis of symmetry is typically vertical.

This type of light distribution is particularly useful in environments where uniform lighting is needed in all directions around the fixture, such as in hallways, stairwells, or certain outdoor lighting applications.

Symmetry

In symmetric light distribution, the distribution of light is symmetric along a plane. This means that if you mirror the light distribution along this plane, the pattern on both sides will be the same. Most often, this plane of symmetry is vertical, splitting the lighting fixture into left and right halves.

This type of light distribution is commonly found in linear lighting fixtures, such as troffers or wall sconces. It is particularly useful in environments where balanced light distribution is needed on either side of the fixture, such as along walls or pathways.

Non-symmetry

In non-symmetric light distribution, there is no axis or plane of symmetry. This means the light distribution varies in different directions without any specific pattern of symmetry. Non-symmetric light distribution is often seen in task lighting or accent lighting applications, where the light is intentionally directed toward a specific area or object.

While this type of light distribution doesn’t provide uniform or balanced lighting, it offers flexibility and precision in highlighting certain areas or creating specific lighting effects. For instance, track lights or spotlights often use non-symmetric light distribution to focus light on artworks, retail displays, or workspaces.

Standard Classifications of Light Distribution Curves

IESNA LM-63 Standard:

Established in 1986 by the Illuminating Engineering Society of North America (IESNA), this was the first industry standard for creating electronic versions of luminaires and light source distribution data. Revised in 1991, 1995, and 2002, the current version (LM-63-2002) has been recognized by the American National Standards Institute (ANSI) and is the only light distribution file format used in North America. The file extension for this format is “*.ies”. It’s starting to be used by some lighting manufacturers and testing institutions in China.

CIBSE TM-14 Standard:

The Chartered Institution of Building Services Engineers in the UK followed the IESNA’s lead, publishing the CIBSE TM14-1988 for the electronic transfer of luminaire photometric data. The revised 1998 version is widely used in the UK.

EULUMDAT Standard:

Proposed in 1990 by Axel Stockmar, a software engineer at Light Consult Inc., Berlin, Germany, the EULUMDAT format for light distribution curves has become the accepted standard for European lighting manufacturers, except for those in the UK. Even without official documentation, this format has gained widespread acceptance. Files use the extension “*.ldt”.

CIE 102 Standard:

Published in 1993 by the International Commission on Illumination (CIE), this standard for light distribution file formats has broad coverage and well-designed formats. However, it has received little support from lighting manufacturers or commercial lighting software development companies and is seldom used.

Other Standards:

There are also other industrial standard formats for light distribution curves currently in use, including EULUMDAT/2 (LCI, Germany), LTLI (Light & Optics, Denmark), TBT (Toshiba, Japan), and CEN (European Committee for Standardization).

How to Use the Light Distribution Curve?

The Light Distribution Curve is critical for understanding how a luminaire emits light, providing essential data for optimal lighting design. It aids in predicting light output direction and intensity, ensuring uniformity, and calculating illuminance levels in a space. Thus, it allows us to choose appropriate lighting solutions to achieve desired aesthetics, energy efficiency, and adherence to safety standards. This understanding is crucial for various applications, from architectural lighting design to task-specific illumination. It provides a visual representation of how light from a specific source will be distributed in space.

Understanding the Curve:

The first step in using a light distribution curve is understanding what it represents. The curve typically shows how the light intensity varies at different angles from the light source. The curve is generally plotted in a polar coordinate system where the angle from the vertical axis (or zenith) represents the direction, and the distance from the origin represents the light intensity.

Choosing the Right Lighting Fixture:

Each lighting fixture has its light distribution curve, which is usually provided by the manufacturer. By comparing the curves of different fixtures, you can choose the one that best fits your lighting needs. For example, a light with a narrow distribution will concentrate its light in a small area, while a light with a wide distribution will spread its light over a large area.

Predicting the Illumination:

Once you’ve chosen a lighting fixture, you can use its light distribution curve to predict how it will illuminate a room. By combining the light distribution curve with the room’s dimensions and the light’s position, you can calculate the light intensity at different points in the room.

Optimizing the Lighting Design:

Using the light distribution curve, you can optimize your lighting design to ensure it meets your needs. For example, you might adjust the position, orientation, or quantity of lights to achieve the desired level of illumination or to minimize shadows and glare.

Simulation and Visualization:

Various lighting simulation software tools can use light distribution curves to create a visual representation of a proposed lighting design. This allows you to see how the design will look and perform before installation, providing an opportunity to make any necessary adjustments.

Remember, using light distribution curves effectively requires a good understanding of light, space, and vision. By combining this knowledge with a careful analysis of the light distribution curves, you can create a lighting design that is both functional and aesthetically pleasing.

How to study the Light Distribution Curve diagram

After introducing the definition and classification, let’s look at something practical – the light distribution curve of the bracket as shown in Figure 2.

Figure 2 Light Distribution Curve (T = C0°-180° A = C90°-270°)

This is the most common polar coordinate light distribution curve. Note the annotation: T = C0°-180° A = C90°-270°; C represents the angle of the horizontal plane (the solid angle is composed of horizontal and vertical angles), and 0°-180° forms a section. T represents the distribution of light on the C0°-180° section (perpendicular to the direction of the lamp tube). Similarly, A represents the distribution of light on the C90°-270° section.

Let’s continue to see how each curve comes about. As shown in Figure 3, the origin of the polar coordinate diagram (at the center of the concentric circles) is the center of the lamp’s luminous surface; each concentric circle represents a light intensity value, with the outermost circle having the highest light intensity; the various angle values in the figure are the vertical angles on that section; the downward direction is defined as 0°, which is done for the convenience of comparing light distribution between different lamps.

Figure 3

Figure 4 below is actually an isoluminant curve diagram, which connects points with the same illuminance as curves. I believe this diagram is easy to understand. Note that this is also a diagram of isoluminant curves in kilo lumens.

Conclusion:

Light Distribution Curves are vital tools in the field of lighting design and analysis. They allow professionals to understand and predict how a lighting fixture will distribute light in space, thereby enabling them to design more efficient, effective, and comfortable lighting systems. From directing light toward specific areas to reducing glare and optimizing energy use, the utility of its is vast. As we continue to advance in technology and the complexity of lighting needs grows, the use and development of more sophisticated and detailed light distribution covers will only become more essential.

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