How to Read a PAR Map for Grow Lights

What is a PAR map

A PAR map is a diagram that a grow light maker includes in the product manual or on the product page. It shows the distribution of photosynthetically active radiation (PAR) across a flat surface, usually a rectangle or square that represents the footprint of the light. The map uses a grid of numbers or a color scale to indicate the PPFD (photosynthetic photon flux density) at each point. For a deeper explanation of PPFD, see the PPFD vs PAR vs Lux article.

The map is measured at a specific hanging height that the maker states, such as 12 inches or 18 inches above the surface. The numbers on the grid are PPFD values in micromoles per square meter per second (µmol/m²/s). A color scale often runs from blue (low intensity) through green and yellow to red (high intensity). The map gives you a snapshot of how much usable light reaches different parts of your plant canopy.

As the University of Minnesota Extension notes, plants need sufficient light for photosynthesis, and the distribution of that light matters for healthy growth. A PAR map helps you see whether a light delivers even coverage or has a strong hot spot in the center with sharp drop-off at the edges.

How to read the numbers and colors

The grid on a PAR map is read like a top-down view of your grow area. The center of the grid usually has the highest PPFD number, because the light is directly below the fixture. As you move outward toward the edges, the numbers decrease. The color scale makes this drop-off easy to see at a glance: a large area of red or orange means a wide zone of high intensity, while a quick shift to blue means the light falls off steeply.

Some maps include a legend that tells you the PPFD range for each color. Others list the average PPFD over the whole grid and the uniformity ratio (often expressed as a percentage or a ratio like 0.8). A uniformity ratio close to 1.0 means the light is very even across the footprint. The DesignLights Consortium, which sets testing standards for horticultural lighting, includes uniformity as a performance metric in its Qualified Products List.

When you look at a map, note the hanging height stated on the map. If the maker provides maps at several heights, you can see how the coverage changes as you raise or lower the light. Always compare maps taken at the same height when evaluating two different lights. For more on how hanging height affects coverage, read Light Distance from Canopy.

What to look for: uniformity and coverage

The two most important things a PAR map tells you are uniformity and coverage area. Uniformity means how evenly the light is spread. A light that has a very bright center but dim corners will force you to place plants only in the center or to rotate them often. A light with good uniformity lets you fill the entire footprint with plants that receive similar light levels. See Light Uniformity Importance for more on why this matters.

Coverage area is the size of the rectangle where the PPFD stays above a certain level. For example, a map might show that at 18 inches, the area within the 300 µmol/m²/s contour is 2 feet by 2 feet. That tells you the light can cover a 2x2 tent at that intensity. If you need to light a 4x4 tent, you may need two such lights or a larger fixture. The How to Calculate Coverage Area guide explains how to match coverage to your space.

Look for a map that shows a gradual, even gradient from center to edge. A sudden drop-off means the light has a narrow beam. A map with a large area of similar color indicates good spread. Remember that the map is a two-dimensional slice; real plants have height and leaves at different angles, but the map is still a useful guide for horizontal coverage.

How to compare PAR maps between lights

When you have two lights you are considering, find their PAR maps in the product documentation. First, check that the maps were measured at the same hanging height. If one map is at 12 inches and the other at 18 inches, you cannot directly compare the numbers. Some makers provide maps at multiple heights; use the one closest to your planned hanging height.

Next, look at the size of the grid. A map that covers a 3x3 foot area tells you the light was tested over that footprint. If your tent is 4x4, you need to see whether the map extends to the edges or whether the maker provides data for a larger area. Some maps show only the area within a certain PPFD threshold; the rest of the footprint may be too dim to measure.

Compare the uniformity ratio if provided. A ratio of 0.9 or higher is very even. Also compare the average PPFD over the footprint. A light with a slightly lower peak but a higher average across the whole area may be better for a full canopy than a light with a very high peak but poor uniformity.

Manufacturers such as Vivosun include PAR maps in their product pages and manuals, allowing you to see the distribution before you buy. GE Lighting also offers grow light bulbs and fixtures; their product pages may include light distribution data. Always read the fine print on the map to understand the measurement conditions. For a list of lights with detailed specs, see the Best LED Grow Lights guide.

Limitations of PAR maps

A PAR map is a laboratory measurement taken under ideal conditions. In your actual grow space, walls, reflective materials, and other lights will change the distribution. The map does not account for light that bounces off tent walls or for the angle of the light as it reaches leaves on the sides of plants.

The map also shows only one plane. Plants grow in three dimensions, and lower leaves may receive much less light than the top of the canopy. Side lighting or multiple lights can improve lower canopy coverage, but a single PAR map does not show that. The article Single Light vs Multiple Lights discusses how to plan for multiple fixtures.

Finally, the map is only as accurate as the maker's testing method. Some makers use a grid of measurement points that is too coarse, missing hot spots or dips. When possible, look for maps from makers that follow industry standards, such as those referenced by the DesignLights Consortium. Use the PAR map as one tool alongside wattage, light type, and coverage area stated in the listing.

Using PAR maps with other specs

A PAR map is most useful when combined with the light's stated coverage area, wattage, and light type. For example, a quantum board light may have a different distribution pattern than a bar light. The map helps you see whether the coverage matches the tent size you plan to use. Compare Quantum Board vs Light Bars for typical differences.

If the listing does not include a PAR map, you can still estimate coverage from the stated footprint and the light type. Bar lights and strip lights tend to spread light more evenly over a rectangular area, while panels and quantum boards often have a more concentrated center. The map, when available, confirms these general tendencies.

For a seed starting shelf, you want a map that shows even coverage across the width of the shelf, with no dark corners. For a 4x4 tent, look for a map that covers the full 4x4 area with acceptable uniformity. The guides on this site compare lights on stated details, including coverage and light type, and you can use a PAR map to refine your choice. See the Best LED Grow Lights for 4x4 Tents guide for tent-specific picks.

What to pick for your job

Questions

What does a PAR map show that a wattage number does not?

Wattage tells you how much power the light draws, but it does not tell you how that light is spread across your plants. A PAR map shows the distribution of usable light (PPFD) across the footprint, so you can see whether the light has a hot spot or even coverage. For more on wattage vs output, see Wattage vs Actual Light Output.

Can I trust a PAR map from a manufacturer?

Most reputable manufacturers provide PAR maps measured under controlled conditions. The accuracy depends on the testing method and the number of measurement points. Look for maps that state the hanging height and the measurement grid spacing. Some makers follow standards from organizations like the DesignLights Consortium.

What is a good uniformity ratio on a PAR map?

A uniformity ratio close to 1.0 means very even light. Ratios above 0.8 are generally considered good for most grow spaces. The exact number depends on the crop and the grower's goals, but higher uniformity means less need to rotate plants.

Do all grow light listings include a PAR map?

No. Many budget or basic listings do not include a PAR map. In that case, you can rely on the stated coverage area, light type, and user guides on this site. Premium lights from brands like Vivosun and Spider Farmer often include maps in their product documentation. Check the Best Samsung LED Grow Lights guide for examples of lights with detailed specs.

How do I use a PAR map to decide how many lights I need?

Look at the footprint that the map covers at your target PPFD. If your tent is 4x4 and the map shows good coverage only over a 3x3 area, you will likely need two lights. Compare the map dimensions to your grow space dimensions to estimate the number of fixtures. The Single Light vs Multiple Lights article covers this in more detail.

Does the color scale on a PAR map mean the same thing across different brands?

Not always. Each maker chooses its own color scale and PPFD ranges. Always read the legend on the map to understand what each color represents. Do not compare two maps by color alone; compare the actual PPFD numbers in the grid.