A parking lot can have high-output LED lights and still have dark areas between parking spaces, along the edges, or near the entrance. The problem is not always a lack of lumens. It can also come from how the light is distributed across the ground.
A lighting photometric diagram helps you see where a fixture sends its light and how much illumination reaches different areas. Instead of choosing a light based only on wattage or lumen output, you can use these diagrams to compare coverage, identify potential dark spots, and understand whether a fixture fits your property.
Three common distribution types are Type III, Type IV, and Type V. Each creates a different pattern, but the type alone does not tell you whether a parking lot will be well lit. You also need to know how to read the diagram.
What Is a Photometric Diagram
A photometric diagram is a visual representation of a light fixture's optical performance. Depending on the diagram, it can show the direction of light, the intensity at different angles, or the illumination expected on a surface.
For parking lot lighting, two types of diagrams are especially useful.
Light Distribution Diagram
A light distribution diagram illustrates the distribution of light from a fixture in various directions. For instance, a polar plot can be used for representing the intensity of light at various angles which is represented by curved lines.
The shape is used to help you understand how a fixture will light the area you are looking at, rather than focusing the light directly forward, how a fixture will project it over a larger space, or how a fixture will cast it around the fixture.
This is helpful in comparing Type III, Type IV and Type V optics. A distribution curve, however, doesn't directly indicate the brightness of each point on the ground. The result is also dependent on the mounting height, fixture orientation and distance.
Illuminance Map or Isofootcandle Diagram
Illuminance Map: provides a qualitative representation of the amount of light reaching the ground over a specified area. It can be represented using contour lines, color gradients or numbers.
Illuminance is typically measured in foot-candles (fc) for the U.S. lighting market. It is measured in metric units as lux. About 10.764 lux is equivalent to 1 foot-candle.
For instance, an illuminance map may have a higher reading close to a light pole and a lower reading away from the light pole. The combination of multiple fixtures results in an overlapping of light to form the overall pattern over the parking lot.
Often, an illuminance map is a better starting point for property owners since it will answer practical questions: Where will the ground be brightest? Are corners adequately illuminated? Will there be shadowed spaces between the fixtures?
For more in-depth information on how to read lighting diagrams, read IESDesk's how to read lighting diagram guide.
How to Read an Illuminance Map
You do not need an engineering degree to understand the main information on an illuminance map. Start with the legend, then examine the coverage pattern and look for areas where illumination drops.
Read the Legend First
The legend explains what the colors, contour lines, or numbers represent. Without it, a bright color does not necessarily mean a particular light level.
Check whether the map uses foot-candles or lux. Then look at the values assigned to the different colors or contour lines.
For example, a map might show 5 fc near a fixture and 1 fc farther away. These values indicate different levels of illumination, but neither value is automatically good or bad for every application. The appropriate target depends on the area, its use, and the applicable lighting criteria.
Also check whether the map represents one fixture or an entire lighting layout. A single-fixture diagram cannot tell you the complete illumination pattern created by multiple poles.
Find the Brightest Areas
Look for the highest readings on the map. These areas often appear close to the fixture or where the light from multiple fixtures overlaps.
A high reading can be useful in an entrance, pedestrian crossing, or another area that needs extra visibility. However, a very bright spot surrounded by much darker areas may indicate uneven lighting.
The goal is not simply to maximize the brightest reading. It is to provide suitable illumination throughout the space.
Look for Dark Spots
Next, examine the areas between poles, along the perimeter, and in the corners of the property.
If the contour lines spread far apart or the readings drop sharply between fixtures, the layout may have uneven coverage. On a color map, large low-illuminance areas can make potential dark spots easier to identify.
For example, a parking lot may appear bright directly beneath each pole but have noticeably lower readings between rows. This can happen when the light distribution, mounting height, or pole spacing does not suit the site.
Before choosing a different fixture, check the entire layout. The solution might involve changing the optics, adjusting pole spacing, changing mounting height, or repositioning the fixtures.
Check the Coverage Area
The outer edge of a light pattern indicates the range of light at the level in the form of a contour displayed. Does not necessarily represent the limit of useful illumination.
The fixture can provide a good spread of light, but may not provide adequate light at the edges. This is why you should base your coverage off of the readings rather than the size of the pattern.
Compare the shape and size of the area illuminated with the layout of your property. Is the light able to reach the ends of the parking rows? Does it include driving lanes? Does the house have corners included?
Bigger doesn't necessarily mean better. It is the intent of providing light where it is needed and that is important.
Check Lighting Uniformity
The uniformity of light distribution over an area. For example, it helps to explain why two parking lots that have roughly the same average illumination can be quite different at night.
A single layout might have a number of areas that are very bright and several that are a bit dim. Another might be able to give the same lighting that's more steady across the same area.
One way of describing uniformity is to compare the average Illuminance with the minimum Illuminance. The average to minimum ratio is the ratio of the average to the minimum, such as the average is 4 fc and the minimum is 1 fc, the ratio of average to minimum is 4:1. In general, the lower the ratio, the more even the light is, but the selection of the target is dependent on the application and the design criteria being applied.
When checking a photometric plan, check the minimum as well as the average reading. One strong average does not automatically mean that the parking spaces/driving lanes are receiving adequate light.
For specific applications, the Illuminating Engineering Society has published guidance in the Illuminance Selector FAQs, including some information on the maintained illuminance and uniformity requirements.
Type III vs. Type IV vs. Type V: What Do the Patterns Tell You
Type III, Type IV, and Type V describe different outdoor luminaire distribution patterns. Their main difference is the shape and direction of the illumination they are designed to produce.
Type III: Broad Coverage for Area Edges
Type III produces a broad distribution pattern that is commonly used for roadways, parking areas, and other outdoor spaces where light needs to extend away from a pole or mounting position.
When reading a Type III photometric diagram, look at how far the pattern extends across the target area. Check whether it reaches the outer parking spaces and driving lanes without leaving gaps farther away.
Type III can be useful along the edge of a parking lot or in layouts where fixtures need to cover a relatively wide area. The actual result depends on the fixture's optical design and orientation.
What to check on the diagram:
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How far the light extends into the parking area.
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Whether the outer spaces receive adequate illumination.
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Whether the light overlaps with neighboring fixtures.
Type IV: Forward-Focused Coverage
Type IV produces an asymmetric distribution that directs light farther forward and across a broad area in front of the fixture. It is commonly considered for perimeter lighting and locations where light needs to extend away from a mounting edge.
On a photometric diagram, look for the direction of the longer portion of the pattern. The fixture's orientation matters because the distribution is not equally balanced in every direction.
For a parking lot beside a building or property boundary, Type IV may help direct light toward the intended area. It still needs to be checked against the actual site layout to avoid insufficient coverage or unwanted light beyond the property.
What to check on the diagram:
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The direction in which the pattern extends farthest.
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Whether the light reaches the intended parking rows.
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Whether the layout leaves dark areas beside or behind the fixtures.
Type V: Symmetrical Coverage Around the Fixture
Type V provides a symmetrical distribution, commonly appearing as a circular or square-shaped pattern in a plan view. It is often considered for open areas where illumination is needed in multiple directions around a pole.
For example, a pole positioned within a large parking area may need to serve spaces and driving lanes on several sides. A Type V distribution can be a suitable option to evaluate because it spreads light symmetrically around the fixture.
However, symmetry does not automatically mean uniform illumination. The actual light levels still depend on the fixture, mounting height, spacing, and overlap with nearby lights.
What to check on the diagram:
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Whether the pattern extends adequately in all directions.
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Whether neighboring fixtures overlap where needed.
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Whether the outer edges and spaces between poles receive sufficient illumination.
Quick Comparison
| Distribution | Typical pattern | Where to consider it | Main thing to check |
| Type III | Broad distribution | Roadways and parking area edges | Forward and lateral coverage |
| Type IV | Asymmetric, forward-focused | Perimeters and edge-mounted locations | Direction and reach of the pattern |
| Type V | Symmetrical, circular or square | Open areas and central pole locations | Coverage around the fixture |
These are general starting points, not fixed rules. A Type III fixture may suit one parking lot layout but perform poorly in another. Always review the specific fixture's photometric data and the proposed installation layout.
For a technical reference, the US Department of Energy's parking lot lighting guide discusses luminaire distribution and explains why the distribution pattern needs to match the lighting application.
What Else Can a Photometric Diagram Tell You
Beyond identifying a distribution type, photometric data can help you evaluate several important aspects of a lighting plan.
Coverage
Check whether the illuminated footprint matches the actual dimensions of your property. Include parking spaces, driving lanes, entrances, sidewalks, and corners.
A diagram with a large footprint may still show low readings near its edges. Review the values at the locations where illumination matters most.
Brightness
Look at the illuminance values across the area, rather than relying only on the fixture's lumen rating.
Lumens describe the total amount of visible light emitted by a source or fixture. Foot-candles and lux describe how much light reaches a surface. A high-lumen fixture does not guarantee high illuminance at every point on the ground.
Uniformity
Compare the brightest areas with the darkest areas. Large differences may indicate that the layout needs adjustment.
If a plan provides an average illuminance value, look for the minimum value as well. These figures offer a clearer picture of how consistently the site will be lit.
Light Spill
Check where the distribution extends beyond the intended area. Depending on the property, unnecessary spill light can affect neighboring buildings, nearby homes, or areas outside the site boundary.
The distribution pattern, fixture orientation, shielding, and aiming all affect where light goes. A suitable optical design can help direct light toward the target area instead of wasting it outside the site.
Pole Spacing and Mounting Height
Photometric diagrams should be reviewed alongside the proposed pole locations and mounting heights.
Increasing the distance between poles may reduce installation costs, but it can also create low-light areas if the fixture's distribution does not provide enough coverage. Mounting a fixture higher changes how its light reaches the ground and can affect the illuminance levels across the site.
A single-fixture diagram is useful for understanding the light pattern. A complete layout calculation is more useful for evaluating the combined performance of all fixtures.
Common Mistakes When Reading Photometric Diagrams
Several common mistakes can lead to the wrong lighting choice.
Looking only at maximum illuminance. A high reading directly under a pole does not show whether the rest of the parking lot is adequately lit. Check minimum and average values, too.
Assuming a larger pattern means better coverage. The light may reach farther but become too weak at the edges. Read the values instead of judging the pattern by its size alone.
Choosing by distribution type alone. Type III, Type IV, and Type V describe general patterns, not guaranteed performance. Two fixtures with the same distribution type can produce different results.
Ignoring installation conditions. Mounting height, pole spacing, fixture orientation, and the number of lights all affect the final result.
Mixing up foot-candles and lux. These units measure the same quantity but use different scales. One foot-candle is approximately 10.764 lux. Make sure the units match when comparing plans.
Treating a single-fixture diagram as a complete lighting plan. The final result depends on how all fixtures work together. Review the combined layout to find potential gaps and excessive brightness.
How to Choose the Right Light Distribution for Your Parking Lot
Start with the layout and the areas you need to illuminate. Then compare fixture distributions and photometric plans against those requirements.
| Site condition | Distribution to evaluate first | What to review |
| Long parking rows or roadway edges | Type III | Coverage along the rows and across the driving lanes |
| Lighting directed forward from a perimeter | Type IV | Forward reach, side coverage, and spill light |
| Open parking area with centrally located poles | Type V | Coverage around each pole and overlap between fixtures |
These examples help narrow down the options, but they should not replace a photometric calculation. The right distribution depends on the exact fixture, pole positions, mounting height, and site dimensions.
Before making a decision, follow these steps:
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Define the target area. Mark the parking spaces, driving lanes, entrances, walkways, and property edges that need illumination.
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Review the fixture's photometric data. Look at the distribution pattern and the illuminance map, not just wattage and lumens.
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Check the proposed layout. Confirm that the pole positions, mounting heights, and fixture orientations match the plan.
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Compare minimum and average illuminance. Look for potential dark spots and large differences across the site.
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Confirm the lighting criteria. Use the applicable project requirements and relevant lighting guidance to determine whether the proposed levels are appropriate.
If you are comparing several LED parking lot lights, ask for the IES photometric file or a photometric layout for your site. These resources provide a more useful basis for comparison than product specifications alone.
Summary
A lighting photometric diagram gives you insight into where the light goes and how it reaches the ground. The distribution pattern gives an overall picture of the light, and an illuminance map can mark out areas of bright light, low light and any potential blind spots.
Type III is typically employed for symmetrical coverage ahead of the fixture and Type V for asymmetrical coverage ahead of the fixture. There is no one “best” selection for all parking lots.
To make a good lighting choice consider the actual illuminance levels, the uniformity, and the overall layout with a mock-up of the pole spacing and height of installation. Looking beyond the distribution label can help you choose a fixture that better fits your site.
Frequently Asked Questions
⏵ What is the difference between a photometric diagram and an illuminance map?
A photometric diagram is a general term used to describe a graphic display of the light performance of a fixture. A polar distribution plot provides information on the change in light with direction, while an illuminance map provides information on light levels on a surface. An illuminance map is particularly helpful for identifying areas in the parking lot that could be too dim or unevenly lit for parking lot planning.
⏵ Which light distribution type is best for a parking lot?
Not all of one type is the best choice for all parking lots. Type III is suitable for broad coverage, Type IV for perimeter applications – forward looking, Type V for open areas with symmetrical coverage. The choice depends on the layout of the property, the height at which the poles are to be mounted, and the distance between the poles, and the desired level of illumination.
⏵ What do the colors on an illuminance map mean?
Colors are used to indicate various light and dark levels, as indicated on the map's key. Colors may represent higher readings, or lower readings, but this depends on the color scheme. When reading the legend, be sure to verify whether the values are in foot candles or in lux.
⏵ Can two lights with the same lumen output have different light coverage?
Yes. Lumen output is a measure of the total light output that is visible, while optical design determines the way this light is distributed. Two fixtures that have the same lumen rating can produce different ground-level illuminance patterns, coverage areas and uniformity. It is better to compare their performance by reviewing their photometric data.
