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:-

  1.   Set the film speed on the camera to 100 ASA or Din.

  2.   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.

  3.   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.

  4.  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