Colour & warming-up data of SOX lamps

Thorn Beta 5 at 'red-orange' state Left: Lighting-up, May 2004, Wrexham.

What colour is sodium light?

Sodium lamps, when fully warmed up, produce deep yellow light of wavelength 589 nanometres. From measuring RGB values at sample points in photographs of SOX lamps the colour seems to be approximately as shown in the box. If anyone knows of a more official RGB value, please let me know!

Red = 250

Green = 230

Blue = 60

(the background colour of this box)

How long does it take a streetlight to warm up? How long does it remain on its initial red colour? On this page I am presenting results taken from my observations which I hope other people will find of interest. Please feel free to make use of my data - all I ask is that I am credited as being the source of it. It's taken a lot of work to put it together!

In October 2007 I carried out my own scientific experiment where I placed a light meter at a distance of 1 metre directly below a Thorn Beta 5 35W SOX lantern, and measured the light level (in lux) every 5 seconds after switch-on. (Note that this was done indoors, so there may be some reflection of light off the walls.) The results are below:

Graph of light level vs. time for typical SOX lamp Summary of results:

Time Light level

0:00 (on) 0

0:30 (red) 1

1:00 (red) 1

1:30 (red) 1

2:00 (red-or) 2

2:30 (orange) 4

3:00 (orange) 7

3:30 (ylw-or) 13
4:00 (ylw-or) 27

4:30 (yellow) 54

5:00 96

5:30 153

6:00 200

6:30 251

7:00 305

7:30 325

8:00 328

9:00 356

10:00 402

11:00 408

12:00 403

15:00 408

The composite photo below shows a typical SOX lamp warming up - a Thorn Beta 5, 1970s/1980s model. The series starts at the time of switch-on and the images were taken at 20-second intervals, the last one being at 7 minutes after switch-on.

Thorn Beta 5 warming up from switch-on to 7 minutes

In the observations I have taken over the years, I have recorded the colour as being one of five possibilities: red (R), red-orange (RO), orange (O), yellow-orange (YO), or yellow (Y). These colours are reckoned by eye rather than by any measuring instrument. The "warm-up" time shows the time taken to reach the final characteristic golden yellow colour, not the time taken to reach full brightness.

In the tables below, the “warm-up to Y” values show how long it takes the lamp to warm up to its final yellow colour from the time it switches on. It will be equal to R + RO + O + YO (allowing a second or two each way for rounding errors). “Sample size” shows the number of days of observations. There is a typical error of a few seconds in the results for each individual lamp, although these should cancel out for multi-lamp averages.

Gresford, 2004 (* denotes Philips MI-26 lamps)

L8*

L10*

M1*

M2*

M4*

All Philips MI-26 lamps (1990s models)

All lamps

1:46

2:08

1:46

1:58

2:27

1:17

1:20

2:08

1:37

1:33

1:55

1:48

0:16

0:46

0:22

0:38

0:13

0:15

0:39

0:26

0:29

0:27

0:27

0:30

0:25

0:38

0:26

0:28

0:26

0:39

0:22

0:40

0:21

0:29

0:30

0:56

0:59

1:12

0:57

1:15

1:06

1:07

1:13

1:48

1:07

1:06

1:10

warm-up to Y

3:27

4:18

3:58

4:23

3:04

3:46

4:10

4:34

3:29

3:56

3:55

Sample size

230

228

229

228

229

227

228

227

230

1141

2286

The analysis of all Philips MI-26 lamps show very little difference from the “all lamps” (average) values, indicating that the statistics do not depend on the type of streetlight: they are more likely to depend on the type and age of SOX tube fitted.

Clitheroe, Oct 1987-Sep 1988

All lamps

2:45

3:00

2:36

2:34

2:47

2:32

2:40

1:56

3:08

2:12

1:25

2:26

1:53

2:29

0:27

0:17

0:26

0:38

0:14

0:09

0:33

0:23

0:12

0:15

0:06

0:23

0:19

0:21

0:25

0:26

0:41

0:28

0:32

0:37

0:41

0:46

0:54

0:49

1:04

0:53

0:39

0:29

0:52

1:17

0:32

0:42

0:57

0:43

0:53

1:10

0:56

1:08

0:38

0:53

0:50

warm-up to Y

4:06

4:35

5:00

4:11

4:15

4:09

4:33

3:53

5:15

4:18

3:27

4:31

3:58

4:19

Sample size

237

227

221

256

234

247

221

237

176

177

142

207

221

2803

The chart on the left shows similar data presented in a different format. The lamps are around Minera, Wrexham, U.K., for 2006-08. Each column of the chart represents a lamp, and each row represents 15 seconds of warm-up time - the two lamps that take the longest to warm up are Philips MAs (probably 135W), with all the others being 35W or 55W. The actual data has been rounded off to the nearest 15 seconds so that whole blocks can be used. Note that the yellow colour presents the final colour of the lamp and not the final level of brightness, which would be reached later.

The graph below is taken from additional data that has been produced.

For a detailed table of warm-up times of streetlights, please click here

I would be particularly interested to know if anyone finds this information useful (so I don't feel I've wasted my time in producing it!). Please let me know if you would like any further detail or explanation.

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