用相机测光（转帖）

 

LIGHT MEASUREMENT    USING A  CAMERA

Many orchid  books mention 'foot candles' when discussing the light requirements required  for orchids.  Sophisticated instruments can be obtained to measure the  light received in a glasshouse or other area, but this equipment is usually  beyond the resources of most orchid growers.  As an alternative you can  obtain a reasonable estimate of foot candles by using a camera equipped with  a light meter.  There is some disagreement regarding the accuracy of  such measurements, but I believe a useful indication of the light levels can  be obtained, accurate enough for most practical purposes.    The procedure is as  follows:-      Set the film speed on the camera to       100 ASA or Din.   Put a sheet of white paper on the       place where you want to perform the measurement so that the paper       receives the highest light intensity.   Point the camera at the paper and       ensure that the image of the paper fills all the view finder so you       obtain a correct reading of the reflected light.  You do not need       to focus your camera. Read the values that the light meter       in you camera gives, and translate the light meter readings to light       intensity in foot candles from the following table.           INDICATIVE    LIGHT INTENSITY     IN FOOT CANDLES         Shutter    speed         Aperture settings                 f 1.4         f 2         f 2.8         f 4         f 5.6         f 8         f 11         f 16         f 22         INDICATIVE LIGHT INTENSITY IN    FOOT CANDLES         1/4 sec         0.5         1         2         4         8         16         32         64         125         1/8         1         2         4         8         16         32         64         125         250         1/15         2         4         8         16         32         64         125         250         500         1/30         4         8         16         32         64         125         250         500         1000         1/60         8         16         32         64         125         250         500         1000         2000         1/125         16         32         64         125         250         500         1000         2000         4000         1/250         32         64         125         250         500         1000         2000         4000         8000         1/500         64         125         250         500         1000         2000         4000         8000         16000         1/1000         125         250         500         1000         2000         4000         8000         16000         32000         1/2000         250         500         1000         2000         4000         8000         16000         32000         64000           EXAMPLE    If you set an exposure time of  1/125 second and the aperture f.stop is f 11, from the table the light  intensity is approximately 1000 foot candles.    This material was originally  produced by the Swedish camera manufacturer Hasselblaad.           FOOT    CANDLE / LUX CONVERSION         F/C     100     200     300     400     500     600     700     800     900     1000     1250     1500     2000     2500     3000     3500     4000     4500     5000         Lux     1076     2153     3229     4305     5382     6458     7535     8611     9687     10764     13455     16146     21523     26910     32292     37674     43056     48438     55820                 Lux     1000     2000     3000     4000     5000     6000     7000     8000     9000     10000     12000     14000     16000     18000     20000     25000     30000     35000     40000     45000     50000                 F/C     93     186     279     372     465     557     650     743     836     929     1115     1300     1486     1672     1885     2323     2787     3252     3716     4180     4645                 Foot-candles divided by 0.0929 =    lux         Lux x 0.0929 = Foot-candles

3.5 What is low light, moderate light and bright light fororchids ?

Let’s start with the maximum light : at noon, on a bright,cloudless summer day, the light level outdoors (in the northeast) will top10,000 fc.

We usually consider the light in a supermarket to bebright.  If we measured that light levelwith a light meter we’ll find it’s intensity is about 200 to 300 fc, and forus, humans, these 200 or 300 fc of light intensity in the supermarket are quitecomfortable.

But 200 to 300 fcs of light is just a pittance for orchids.

To grow and bloom properly :       Low end      High end

• Low light orchids need           1,000 fc     1,500 fc

• Moderate light orchids need      1,500 fc     3,000 fc

• High light orchids need          3,000 fc     4,500 fc.

Please keep in mind these levels of light are for matureplants and the maximum light is for a limited time at the brightest moment ofthe day (noon in a bright, cloudless, summer day).

Please also keep in mind you can acclimate some of yourorchids to grow a little outside of these ranges, within reason of course.

The book “An Introduction to Orchids - A Guide to theGrowing and Breeding of Orchids” published by The South Florida OrchidSociety(Tel 305-226-4757) and the book “All About Orchids” published by OrthoBooks both have very clear charts about light requirements of orchids.

3.6 Where to grow you orchids at home

In practical terms what this means is that to provide theminimum light our orchids need to grow and bloom we’ll have to grow them :

Low light orchids

-on the windowsill of an east window

-or within 1 foot of a west windowsill

-or within 2 feet of a south windowsill

-or within 1 foot of a south windowsill if it is shaded byother plants or by shade.

Medium light orchids

-on the windowsill of a west window

-within a foot of a south windowsill

High light orchids

-on the windowsill or within a foot of a south windowsill,

(as long as they donot get direct sun in the middle of the day).

Please keep in mind we are talking about windows that are not shaded by trees or other buildings.

文学城rokkorhunter写的：
拿出相机来，选择光圈优先（Aperture priority），ISO设100，光圈值定在16，爆光补偿exposure  compensation设在0。这样你的像机就能当测光表用了。找个大晴天，让太阳光直射在你的手上，用像机对着手充满全画面，半按快门，这时像机测光表给出的快门数。应该在1/100秒左右，人的肤色有深浅，不同像机也可能有点差别，读数可以在1/50到1/200不等，不管你的像机读数是多少，假定是1/100，记住这个值。它就是太阳照到地球的光照强度，相当于EV16。冬天稍低点，夏天稍高些，雪天或沙滩反射强也会高一些。地球离太阳的距离没有大幅变动，这个光照值晴天相当稳定。阴雨天才会有显著降低。万物生长靠太阳，植物要是每天能有七八个小时这样的照度，就能种各种粮食水果。蝴蝶兰生长在热带雨林的树干上，一般没有阳光直射而是生长在树荫下。这样的照度，用像机测的话，得到的快门值大概在1/10到1/25秒，是1/100秒的10倍到4倍。也就是说，树荫下的光照度，只有全日照下的1/10到1/4。冬天阴天时我家离窗户一米之外，，测光给的快门值常常是1到2秒，也就是阳光下的百分之一不到！难怪人家要休眠。不过好在兰花并不像西瓜番茄需要全日照，前面说了1/10到1/4相对日光强度是蝴蝶兰正常生长条件，如果灯光能达到这个照度就好了。


这是我给兰花过冬提供的两只4尺日光灯，伸手离灯10厘米左右，测出快门1/25秒，20厘米，1/10秒。灯光跟太阳光的一大区别，灯光下光照强度，跟光源的距离平方成反比。离灯过远，照度就极度下降，所以大型植物如种大麻的，用日光灯是不管用的，但兰花小型草本就没问题。所以促花的第二个要点，把兰花叶片放在灯下10到20厘米的范围，刚好满足兰花的理想光照。这样冬天兰花保持快速生长，花期很容易提前两个月。