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Mt. Vernon, WA 98273
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6639 Light Meter (N3639)



Create the optimum light for your plants’ growth

Light is the most important factor affecting plant life. Using this meter, you can determine and accurately adjust the amount of light for each of your plants, creating optimum growing conditions for them. The meter measures in footcandles light emitted from fluorescent and HID fixtures, and from the sun. A handy sensor on a 24" cord lets you easily measure under lights and in foliage. It is powered by a photovoltaic cell – no batteries to wear out. Lighting booklet with light intensity and exposure times for over 150 species included. Range 0 to 10,000 footcandles.

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Learn More About
6639 Light Meter (N3639)

General Product Information:

The Light Intensity Meter is designed specifically for the measurement of radiant light energy that directly affects plant growth, development and seed germination.

The meter is not color corrected. With this feature, we can assure maximum accuracy in measuring the intensity of the light emitted from the sun and can compare the light intensity of artificial light sources to the proper radiant color of the sun because the sun is the standard light color that affords plants maximum growth and development.

The meter incorporates a silicon photovoltaic cell that converts radiant light energy into electrical energy alleviating the need for a battery. The photovoltaic cell exhibits a very fast reading, and has a stable, linear and reproducible out put per light intensity. The cell displays no fatigue and has unlimited life expectancy. It has versatility and assures freedom from personal operator error factors. (the photovoltaic cell used is superior to light meters using selenium cells in efficiency, stability and lack of fatigue).

The meter is a pocket-sized and portable, light-sensitve instrument that is capable of reading up to 10,000 footcandles. It can be used to determine brightness, reflectance, transmittance and the most important measurement, illuminance.

The logarithmic response of the meter provides accurate meter deflections and is easily read at all light intensities.

The meter is permanently calibrated to footcandles and the scale is divided into 4 categories of light intensities preferred by all plants.

A clear epoxy dome is affixed over the photocell as a permanent protective feature to assure maximum durability.


  • Instant readings, permanently calibrated.
  • Lifetime silicon photocell.
  • 0 to 10,000 footcandle range.
  • 2½ ft. extended sensor probe.
  • Measures sunlight, Fluorescent, Metal Halide and other High Intensity Discharge (HID) light.
  • Accurate to 2%
  • No batteries required.
  • Comes with a informative book complete listing of light intensities and exposure times for over 150 plant species.

Shipping Information:
Standard shipping charges apply.

Operating Instructions

To measure illuminance: To determine the level of illuminance on the surface of a leaf, point the universal sensor probe directly at the radiant light source while holding the probe at leaf level. Avoid positioning your hand or body in such a way as to inhibit or reflect the light.

To measure Luminance: On transmitting surfaces, hold sensor probe directly on the surface. On the reflecting surfaces, hold the sensor probe approximately two inches above the surface.

To measure Reflectance: Make an illuminance reading, then make a luminance reading as described for reflecting surfaces. The ratio of reflected footcandles to illuminance footcandles is the reflectance.

To measure Transmittance: Place the tip of the sensor probe directly under and “flush” with the leaf surface, making sure that light source is on top of leaf. Take reading and note. Now take an illuminance reading at leaf level. Note or jot down reading. The rate of the first reading to the second reading is transmittance or translucency of the leaf.

With the transmittance measurement you can closely predict how much light a leaf absorbs, how much is reflected and how much goes directly through the leaf (transmittance). The efficiency of photosynthesis for your plant can now be evaluated.

Take illuminance readings in the early morning at noon and again in the late afternoon to determine the average footcandles your plants are receiving. The average footcandle multiplied by the time of exposure will yield the footcandle hours.

Clarification of Readings:
Sun & HID: Slide switch up. Read top scale ONLY. Multiply all readings by 1000.
Fluorescence: Slide switch down. Read bottom scale ONLY. Multiply all readings by 10.

For plants already in position.
Note: Light Meter has three positions: up- Sun & HID, middle- Off, down- Fluorescent

  • Measure the light intensity at leaf level.
  • Consult the Table of light requirements (in instruction booklet)
  • If too much light:
    • Reposition plants.
    • Shield plant
    • Screen or curtain light
  • If too little light:
    • Reposition closer to light
    • Reposition to another location
    • Supplement artificially
  • For New Plants:
    • Take readings at all possible locations and at different times of the day.
    • Take readings at several distances from the light source
    • Jot down readings of each location and distance.
    • Consult table of light requirements (in instruction booklet)
  • For further information, please contact Charley's Customer Service at 1-800-322-4707.

    This item is backed by Charley's 100% guarantee. If you are not completely satisfied, return your purchase in the condition you receive it, within 30 days, for a refund, exchange or credit - whichever you prefer. If any product is found to be defective within the first year, it may be returned or exchanged for full credit.