Introducing the PAR Meter

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Light is important to orchids (and plants generally), being the energy source for photosynthesis and thus plant life. But to use a variation of a popular saying, 'light ain't light'. Besides the quantity, light must be of the right type (or spectrum range) for successful orchid growth and flowering.

All orchids do not require the same amount of light. Some orchids grow best in full sunlight, while others require heavily shaded situations. While these general descriptions are helpful, better quantification of light requirements by orchid type and the ability to measure the amount of light available in our orchid houses would be useful. How often have you observed that by moving an orchid from one position to another in the orchid house makes a big difference to the growth of the orchid. Also, have you ever considered the effect of different covering materials on light transmission?

A useful instrument for measuring the amount of light available which can be utilised by plants is a Photosynthetically Active Radiation (PAR) meter. This measures the amount of light in the wavelength of 400 to 700nm (nano metres = 10-9x1 metre) in falling on a flat (level) surface. It recognises that as the angle of the sun in the sky declines (going into winter) the sunlight is spread over a wider surface area (think of a ray of sunlight as a drinking straw and how the cross-sectional area of the straw increases as it is cut at more acute angles).

The PAR Meter

The society has invested in a PAR meter which is available for use by all members (contact Jane Wright). An image of the meter is shown below:

The meter consists of two main components:

Operation of the meter involve holding the sensor vertically and pressing and holding the button on the reader until numbers appear on the screen. It is important that the sensor be held vertical while taking a reading.

The meter provides measures in terms of μmoles/sec. For all practical purposes the dimensions of the reading are unimportant. What is important is that the maximum reading you can expect is about 2.4 in full sunlight.

You will find that readings are quite sensitive and it is best to take several readings in exactly the same position as quickly as possible and take an average of those readings (even discard any that are 'way out'). I suggest it is best to take readings when the sky is clear (cloudless) as even small changes in cloud cover have quite an impact on the readings. Readings will obviously vary by time of day, day of the year and of course cloud cover.

Some results

At this stage we are finding our way in the use of the meter. There are many issues requiring further investigation, such as whether an average should be taken over the day (and if so, how frequently should readings be taken), how to handle readings in different seasons and the importance of radiation outside the PAR band (especially ultraviolet radiation).

The following chart provides illustrative readings I have taken to investigate the effect of various orchid house covering materials on light availability. The materials were simply spread over a frame held level and readings taken approximately 6 inches below the material. Readings were taken hourly in mid February over various days in order to get readings under clear sky conditions.

Readings are shown in the chart for full sun, agricultural plastic, glass (3mm), aged polyflute and 50% shade cloth. In general, glass and ag plastic had, as might be expected, the least reduction in the amount of light. I also took several measurements using single layer clear polycarbonate (SUNTUF®) which gave similar readings to glass and ag plastic. I also tried different coloured shadecloth (cream and green) but there was no noticeable difference (guess this might be expected as both were rated at 50%).

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