The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR
. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).
Color Rendering Index
In general, CRI is a numeric indication of a lamp's ability to render individual colors accurately. The CRI value comes from a comparison of the lamp's spectral distribution to the standard (e.g. a black body or the daytime sky) at the same color temperature. The higher the CRI the more natural and vibrant the colors will look. A bulb with a CRI of 85 or higher is excellent being that the sun has a CRI of 100.
Eye Hortilux makes 90 -92 CRI bulbs that are used in aquarium, horticulture and other applications.
Color Shift and Variation
Different colors are produced in metal halide lamps by using various arc tube shapes and metal halide salts. In new lamps these halides need to "burn-in" for approximately 100 hours before they reach their optimum color. This is why new lamps can sometimes be unstable or vary in color.
a mh halide as a cri of around 85 a hps has a cri of 24
hps isn't so good for veg, i don't get anywhere near as tight node spacing with a hps as i do with a mh