Why HPS Lamp Lumens Cannot Be Compared Directly With LED Lumens

The 7,000-lumen HPS lamp results in approximately 3,640 net lumens after fixture and photometric losses, while the LED example provides 4,400 net lumens

When reviewing an HPS-to-LED replacement, one of the first checks is whether the lumen figures in the two datasheets describe the same thing. A lamp rating and a complete-luminaire rating are not automatically equivalent.

This distinction matters during preliminary selection because a direct comparison can lead to an unsuitable LED output target. However, calculating a lower estimated HPS output does not, by itself, prove that a particular LED replacement will deliver equivalent lighting.

Lamp output and luminaire output

A high-pressure sodium (HPS) lamp datasheet may state the luminous flux produced by the lamp under specified test conditions. The complete luminaire has its own optical system, which affects how much light exits the fixture and how that light is distributed.

For a conventional lamp-and-reflector system, luminaire output can be lower than lamp output because of optical losses. The actual relationship depends on the fixture design, condition, lamp and the data available for that installation.

LED datasheets commonly state luminaire luminous flux, but the basis should still be checked. Do not assume that every figure labelled “lumens” uses an identical measurement method.

How to compare the figures correctly

Where reliable data are available, use the measured or declared output of the complete existing luminaire and compare it with the declared output of the proposed LED luminaire. If only the HPS lamp rating is available, any estimate of the complete fixture output must identify its assumptions.

A simplified calculation may be written as:

Estimated luminaire output = lamp luminous flux × luminaire efficiency

This calculation is valid only when the efficiency factor is appropriate to the fixture and is defined consistently. Do not apply another loss factor if that loss is already included in the efficiency or output figure.

Illustrative example: 70 W HPS

Assume an HPS lamp is rated at 7,000 lm and an illustrative luminaire efficiency of 65% is used. If that efficiency represents the complete optical efficiency from lamp output to luminaire output, the estimate is:

7,000 lm × 0.65 = 4,550 lm

The result is an illustrative estimate of luminaire output, not a verified value for every 70 W HPS fixture. If the actual fixture photometry is available, use that data instead.

Do not apply an additional 20% loss automatically. First establish whether that loss represents a separate, justified effect or whether it is already accounted for in the efficiency or output data.

Why the original lamp rating is not a replacement specification

Suppose the existing lamp is rated at 7,000 lm. Specifying an LED luminaire at 7,000 lm solely to match that number may be inappropriate if the HPS figure is lamp output while the LED figure is complete-luminaire output.

The reverse is also important: an estimated HPS luminaire output does not establish the LED output required for the project. The replacement must still be checked against the actual lighting task, installation geometry and design criteria.

What the procurement reviewer should request

• Existing HPS lamp datasheet and rated luminous flux.
• Existing luminaire model and available photometric data.
• Clarification of whether each stated lumen value refers to the lamp or complete luminaire.
• Proposed LED luminaire datasheet and its declared luminous flux.
• Photometric files or lighting calculations where required by the project.
• Confirmation that the proposed luminaire meets the hazardous-area specification and certification requirements.

Lumens do not prove equivalent lighting performance

Even when both figures represent complete-luminaire output, similar lumen values do not guarantee the same illuminance at the work plane. Optical distribution, mounting height, spacing, orientation and the geometry of the area affect the result.

Where the project specifies an illuminance requirement, the proposed arrangement should be evaluated against that requirement using suitable photometric data and the applicable design criteria.

Hazardous-area suitability is a separate check

An LED luminaire may provide suitable lighting performance but still fail the hazardous-area requirements. Verify the complete Ex marking, certificate scope, required protection concept, gas or dust group, EPL or zone suitability, temperature limits, ambient range and project-specific conditions.

Likewise, Ex certification alone does not demonstrate that the installation will achieve the required lighting performance. Both checks are necessary.

Conclusion

Do not select an LED replacement by matching the HPS lamp lumen rating alone. First establish what each lumen figure represents and use actual luminaire data wherever possible. Then verify the proposed lighting arrangement against the project's illuminance and hazardous-area requirements.

This article explains the comparison basis. For the calculation and limitations of a specific 250 W example, see Replacing 250W HPS: How Much LED Output Do You Actually Need?

For evaluating illuminance and the lighting design at the work plane, see Why LED Retrofit Should Start With Lux, Not Lumens.

Technical references

• Existing and proposed luminaire manufacturers' datasheets and photometric files.
• Project lighting specification and applicable lighting-design criteria.
• Applicable hazardous-area standards, project specifications and certification documents.

Note: The numerical example is illustrative. Confirm the efficiency definition and actual fixture data before using a calculation for engineering acceptance.