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Blue line represents LED average wavelength (based on Blue LED pump) however, white light is a broad band over many (theoretically all) wavelengths. Measuring devices follow the photopic curve. Human eye sensitivity response in daytime follows photopic curve. Human eye sensitivity response in nighttime follows scotopic curve. The HPS light tested by a meter (photopic) to 10 units. The same HPS light viewed by the human eye at night will see only 20% (where yellow line crosses dotted curve) at night. Result eye sees say 20% of light. The White LED tested by a meter (photopic) to 10 units. The same White LED light viewed by the human eye at night will see 6 units. Result, eye sees 60% of light. This is actually higher since the broad band would achieve more like 80% visibility if not 100%. The HPS needs to be 40%-60% brighter than White LED to have similar visibility. This is why the white LED light can be half the measure value with a similar lighting effect as viewed by the human eye.
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