Measuring Light

Virtually all light is measured in foot-candles, lux, or lumens. Foot-candles and lux measure light visible to the human eye. The human eye sees much less of the light spectrum than plants "see." The eye is most sensitive to light between 525-625 nanometers. The importance of the blue and red portions in the spectrum is diminished greatly when light is measured in foot-can-

Although this simple light meter measures light in foot-candles rather than PAR, it still gives an accurate idea of light distribution.

dies, lux, or lumens. A foot-candle is a unit of illumination equal to the intensity of one candle at a distance of one foot. The lux scale is similar to that of the foot-candle; one foot-candle is equal to 10.76 lux.

Humans see light differently than plants do. Compare the graphs above to see how the light you see differs from the light a plant uses to grow. Plants use the photosynthetically active response (PAR) portion of the spectrum. Humans use the central portion of the spectrum, while plants are able to use large portions of the spectrum not measured by light meters that record foot-candles, lux, and lumens.

Light is also measured in spectrum with Kelvin temperature which expresses the exact color a bulb emits. Bulbs with a Kelvin temperature from 3000 to 6500 are best for growing marijuana. The PAR section above explains that plants use specific portions of the spectrum-a complete range from blues to reds. Lamps with a spectrum similar to PAR-rated bulbs can use Kelvin temperature of a bulb to ascertain the approximate PAR rating ol the lamp. Color spectrum results from a specific mix of different colors. High intensity discharge bulbs are very similar in spectrum. Making these safe assumptions, a rough PAR rating could be extrapolated from a Kelvin temperature rating.

The Color Corrected Temperature (CCT) of a bulb is the peak Kelvin temperature at which the colors in a bulb are stable. We can classify bulbs by their CCT rating which tells us the overall color of the light emitted. It does not tell us the concentration of the combination of colors emitted. Companies use a Color Rendering Index (CRI). The higher the CRI, the better the bulb is for growing.

Light Meters

Most commercial light meters measure light in foot-candles or lux. Both scales measure light to which the human eye reacts to "see.'1 They do not measure photosynthetic response to light in PAR watts.

Light measurements in this book are made in

Bulb

Model

Mfrg

MH/HPS Watts

Initial

Color Deg.

Lumens

K

Sunmaster

Warm Deluxe

V

MH

1100

133000

385 PAR

AgroSun

AgroSun

V

MH

1000

117000

3250

Multivapor

HO

GE

MH

1000

115000

3800

MultiMetal

Super

1

MH

1000

115000

4200

Metal Halide

Metal Halide

Ph

MH

1000

110000

3700

Solarmax

Veg

V

MH

1000

85000

7200

Super Metalare

Super

0

MH

1000

1)5000

3600

Sunmaster

Warm Deluxe

V

MH

1000

117000

315 PAR

Sunmaster

Natural Deluxe

V

MH

1000

117000

315 PAR

Sunmaster

Cool Deluxe

V

MH

1000

80000

315 PAR

Solarmax

Veg / Conversión

V

MH

600

55000

7200

Solarmax

Veg

V

M H

400

32000

7200

Sunmaster

Warm Deluxe

V

MH

400

40000

110 PAR

Sunmaster

Natural Deluxe

V

M H

400

40000

no PAR

Sunmaster

Cool Deluxe

V

MH

400

32500

110 PAR

Super Metalare

Super

0

M H

400

40000

4200

Super Metal Halide

Super

Ph

M H

400

4000

4300

AgroSun

AgroSun

V

MH

400

4000

3250

Multivapor

HO

GE

MH

400

40000

4200

Sunmaster

Warm Deluxe

V

MH

250

22000

85 PAR

Sunmaster

Natural Deluxe

V

MH

250

23000

85 PAR

Sunmaster

Cool Deluxe

V

MH

250

21500

85 PAR

Super Metalare

Super

0

MH

250

23000

4200

Super Metal Halide

Super

Ph

MH

250

23000

4300

Multivapor

HO

CE

MH

250

23000

4200

Hortilux

Super

HPS

1000

145000

2100

Solarmax

Super HPS

V

HPS

1000

147000

2100

Lucalox

HPS

GE

HPS

1000

140000

2100

Sunlux

HPS

t

HPS

1000

140000

2100

Lumalux

HPS

0

HPS

1000

140000

2100

Ceramalux

HPS

Ph

HPS

1000

140000

2100

Solarmax

Super l-IPS

V

HPS

600

95000

2100

Sunmaster

Super HPS Deluxe

V

HPS

600

85000

358 PAR

Lumalux

Super

0

HPS

600

9000

2200

SonAgro

Plus

Ph

HPS

600

9000

2100

Hortilux

Super

1

HPS

430

58500

2100

Hortilux

Super

1

HPS

400

55000

2100

Solarmax

Super HPS

V

hi PS

400

55000

2100

Lucalox

HPS

GE

HPS

400

41000

4000

Sunlux

HPS

1

HPS

400

50000

2100

Lumalux

HPS

0

HPS

400

50000

2100

Ceramalux

HPS

Ph

HPS

400

50000

2100

Hortilux

Super

1

HPS

250

32000

2100

Lucalox

HPS

GE

HPS

250

30000

2100

Sunlux

HPS

1

HPS

250

29000

2100

Lumalux

HPS

0

HPS

250

29000

2100

Ceramalux

HPS

Ph

HPS

250

28500

2100

foot-candles and lux. This information is still valuable, because it records the amount of light spread over a specific surface. The information is then coupled with the PAR rating of different bulbs. Regardless ol the lamp, the amount of light emitted is constant. It only makes sense to use the proper reflective hood with a high PAR-rated bulb to grow the best garden.

After all the talk about PAR watts, industry officials are unable to agree on a common scale of measurement. For this reason, we have decided to rely on Kelvin color temperature to measure lamp spectrum.

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Growing Soilless

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