|
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 |
|
