# LUX, PAR, PUR and DSLR



## tmiravent (5 Jan 2014)

Hello,

I have some questions about the light.
I believe many of us have the same issues, the first doubt is: _Is the light the best possible?_

About a year restarted the hobby and quickly became a fan of the LED .
Now i'm 100% LED (nothing against other lights) is just my preference.
But i find sometimes lack of knowledge about this type of light, works with some but seems to not very good with others...
To learn more we need to measure, that's one way to go (we can also choose the try and error track).
All things are possible to be measured and compared , is too complicated?
After reading and researching some articles , especially on LED , I have some questions that some experienced 'users' may be able to respond .

*Luxmeter [convert LUX to PAR ? ]*
A simple light meter (lux meter) will not take accurate readings of aquarium lighting. There is always a error involved. Reading some articles and forums come to the conclusion that it is possible to measure with some accuracy , if we know the correction factor. It seems that the error is constant and if we know the factor it will give useful values.
So, in practice, despite the limitation of the light meter to the green (most important in view of the human being) region it is possible to assess a value to convert ( implies knowledge of the specific features of the lights that we measured ) .

Question 1 : _The conversion works with sufficient accuracy ?_

*PAR*
After digging about PAR, types of meters , handset prices and characteristics of those devices, there was a little bug scratching my ear...
The measurement of PAR value (active radiation available for photosynthesis in plants ) is more relevant to know the light that can be used in photosynthesis , and the measurement PUR is this part that is actually used by plants or specific plant . Very interesting and important to know and understand the concept PAR | PUR .
But the PAR measurement is used mainly to study contexts \ biological and agricultural controls . Important to know the specific conditions ( amount of light throughout the day / week / year) to calculate the profitability of crops , for example.
But the aquarium ( in principle) the luminous conditions are stable , we have no clouds , storms , sunrise / sunset and the seasons . The variation is controlled by us , artificially . The results are expected to be consistent.

Question 2 : _Is the PAR meter the best way to meet the amount of light for the plants ?_

*PAR meters*
This detail is very interesting as devices are generally ' relative 'expensive.
But the characteristics of these devices did not seem very special ( maybe the price is partly justified by the very small market ... ) . Perhaps my ignorance, but did not seem too complex by the way it works or in construction...
Some 'users' connect the sensors to a standard multimeter achieving a reading for PAR (with correction factor) .
But the sensors themselves ( those that capture photons ) seem somewhat basic (maybe I'm wrong ... )

Alternatives ? But what alternatives have a PAR meter ? a multimeter ? an altered light meter ?

Question 3: _Can we use the sensors of our cameras ?_

These have a recording capacity of the entire light spectrum .
(in truth even has the ability to see beyond - infra and ultra but are fitted with filters to bock them, but that's another story ... )
I suspect that PAR has no meter as accurate and rigorous as that sensor have in our DSLR . I also suspect that the analysis of RGB curves captured by the machine can provide us with great accuracy the behavior of each lamp and lighting setup that we have in each aquarium.
I know that the readings 'Photos' favor ( the most important for our vision and photography) green channel at the expense of others, but we have the 3 channels at one shot.

A simple guide for everyone to read the PAR values of the aquarium with a digital camera would be a big improvement. Can it be done? I believe it can.

Question 4 : _Has anyone done this? ( if yes please tell me ) Is this a daydream ?_


*SIMPLE GUIDE*
*DSLR Lux reading [edit: use with caution]*
Here is a simplified formula to read LUX with your digital camera (DSLR).
I've made the comparison with a lux meter and seems to work.

_Use a white board (most white possible) White Card __90%__ reflectance_

*LUX = (**62**x**f**2**) / (**t**x**ISO**)*

*62 conversion* factor for white paper
*f **diaphragm*
*t* time/velocity
*ISO *sensibility

Example for a typical situation with: f 8 | t 1/60 | ISO 100

(62 x82) / (1/60x100) = (62 x 64) / ( 1,7) = 3968 / 1,7 = 2334 LUX


A good year for all (full of light , with a perfect PAR with a calibrated spectrum for plants)
tiago
_(Sorry for my English errors)_


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## tmiravent (5 Jan 2014)

Sources
Clarkvision: Articles 
PAR Readings - what are good readings ? | UK Aquatic Plant Society
Breaking rules | UK Aquatic Plant Society
Par meter recommendations | UK Aquatic Plant Society
 DIY PAR meter, Yeah you heard me
 DIY PAR meter - Reef Central Online Community
Apogee Instruments Quantum Sensors and PAR Meters - Measuring PPF, PAR and PPFD
Aquarium Equipment: PAR Meters and LEDs - How Accurate are the Measurements? A Comparison of Three Meters and Lux to PAR Conversion Factors for LEDs — Advanced Aquarist | Aquarist Magazine and Blog
There are a lot more.


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