Growing under lights

[Femme]

I just figured out how many fc my lamps give.
I have 2 100W HPS in a old fishtank ( wich is 60cm high)

12,5cm from the lamp it is 8700 fc
25cm from the lamp it is 4400 fc
37,5cm from the lamp it is 2900 fc
50cm from the lamp it is 2200 fc
and on the bottom it is about 1800 fc

If you want to know how much light your lamps provide for your plants, this a way to calculate:

First you need to know how many Lumen your light give
Take the amount of Lumen and multiply with 45,84
(For ex. 100W=9500 Lumen, so 9500 x 45,84= 435480 )
Take that sum and divide with the distace from the lamp to your plant in inch ( in my case 435480/10 inch= 43508)
Remove the last number and you get the result of how much your lights give your orchids (in my case 10 inch from the plants give 4350 fc )


Or am i totally wrong?

[camille1585]

I don't think that works using lumens as the basis of your calculation, but i'm no light expert. Lumens is the total amount of light emitted by the light, in every direction. However, when calculating how much light your plants get, you're only interested in the light hitting the plants. Even with a good reflector you will never have all the lumens going in one direction. Footcandles and Lux represent the amount of lumens falling in a certain area (1 square foot or meter). I think you're better off getting a good light meter to know what your plants are getting!)

Where do you get the number 45,84 from?

[Magnus A]

Hi Femme

I think there is something wrong with the calculation. I base this on these value:

12,5cm from the lamp it is 8700 fc
25cm from the lamp it is 4400 fc

Due to those the intensity is half when the distance is dubbled but the light intensities falls with the distance squared (distance*distance). This would result in roughly 2200 fc at 25 cm if 8700 fc at 12,5 cm is correct.

As Camille I wonder from where do you get the number 45,84 from?
I also wonder the reasoning for the statement "Remove the last number and you get the result"

/Magnus

[nhman]

Individual bulbs will vary per bulb, per company, and per the variance of the electrical power at any one point.
Yet another factor to consider is that as ANY light ages, the output will decrease.
I would recommend the purchase and use of a good light meter. This is simple and accurate way of measurement in "real time".
This way you can monitor the output and the expected decrease in light production with the age of the bulb(s) and decide when the best time to change the bulb(s) would be.

[Femme]

My calculation is based on 100W Philips ZonT+ HPS and a 120 degree angle reflector (and that lamp gives 9500 lumen)
I will give you a complete calculation tomorrow

[Magnus A]

Hi Femme

I did not see that you were from Sweden when I wrote my last comment.

You can get a deasent light meter at Clas Ohlson for 298 SEK, Clas Ohlson Internetbutik - Allt för en enklare vardag . I have one and I compared it with a calibrated light meter at work and it was acctually quit spot on!

[Femme]

Thanks Magnus!
Why i havent got a lightmeter is coz i have read that the lightmeters you can buy "cheap" really suck and that the accurate ones (PAR) cost about 10.000 SEK
But if you recommend this one, i will buy it for sure

[Magnus A]

Femme,
You are right that a cheap light meter may not show the absolute correct value, and they can not be calibrated. But thay are relative and by meassure the light under your light with regular intervals, for example once a month, you will see when the bulbs start to lose power and thereby you will now when it is time to change them.

And Yes, for an accurate scientific calibrated light meter, that only measure "PAR" and do this uniform in that light region (400-700 nm), They ARE expensive.
The one in the link above is not a PAR calibrated meter but it is good enough for home use to get a feeling.

//M