Interior plant growth depends upon both the quantity and quality of natural light transmittance.  The quantity of photosynthetically active light is measured by the PAR (Photosynthetically Active Radiation).  The primary PAR response occurs well within Heat Mirror's visible transmittance curve. Because of this high PAR transmittance, Heat Mirror glazing applications promote thriving plant growth.

Light quality depends on the ability to selectively transmit useful radiation (visible light and PAR) while controlling thermal stress and damaging radiation (infrared thermal burn, ultraviolet radiation). Heat Mirror's wavelength-selective coatings provide the means for improving light quality.  The exact choice of Heat Mirror type depends on building type, orientation and climate.

Heat Mirrors 88 and TC88 provide the maximum PAR transmission for northern, cold-climate applications where thermal burn problems are not anticipated.  Heat Mirrors 77, 66, TC88 and SC75 render the best PAR vs. IR-UV ratio for optimal light quality and growth rates; use HM 77 and TC88 in northern climates and HM 66 and SC75 in the southern climates or anywhere excess heat gain is a problem.  Heat Mirrors 55 and 44 have slightly lower PAR curves yet still provide much higher growth than commercial reflective glazings.  In general, use clear glass with HM 88, 77, 66, and SC75 in vertical applications while reserving HM 55 and 44 for sloped usage with either clear or tinted glass.

A few plants species (about 2%) have "far-red photomorphogenesis" requirements in the area of 700+ nm (these wavelengths are in the boundary region between visible light and near infrared).  Such plants types generally do not grow well behind any glass unless supplemented with artificial light sources.  Incandescent (not fluorescent) lights may be used to promote development for these plant varieties.

We often receive questions regarding plant sensitivity to ultraviolet light (below 380 nm). According to the Denver Botanical Garden's* Horticulture Department and published reference materials, no ultraviolet light requirements have been documented for plant growth.  High intensity artificial UV light is occasionally used by the Denver Botanical Gardens to sterilize soil and plant samples prior to planting.

In summary,  clear Heat Mirror units promote optimal plant growth by transmitting photosynthetically active radiation and controlling excessive temperature swings and thermal stress.

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* Denver Botanical Garden, 909 York Street, Denver, CO  80206; phone 303 331-4000.