[JB]

In most lighting design software available today there are user-defined or
predefined surface reflectance values for many commonly used materials and
surface finishes found in building interiors.
In typical office lighting designs the 'default' values used in the UK seem
to be 70:50:30 or 70:50:20 for Ceiling/Walls/Floor. This I suppose, is meant
to represent typical matt or satin white paint for ceilings, white or light
coloured paint on the walls, and some darker carpet for the floor.
In order to get a more accurate value for real reflectances of actual
surface finishes or materials, what instrument do you use to measure this in
the 'real-world'?
I know I can use my spectrometer with a calibrated light source and standard
certified reflectance target (like Labsphere Spectrolon etc.), but this is
not the sort of kit you want to be carrying around for casual
spot-measurements on site.
Are there any portable or handheld instruments on the market which can be
simply held against a surface to get a reading of total diffuse reflectance?
If not, I'm thinking of building one for our own use, possibly based on a
group of RGB leds or even a tungsten lightsource and a detector such as the
Taos TSL230 or TCS230.
Anyone use anything similar or have any ideas of 'homebrewed' instruments
they use in the field?
Any thoughts on this subject would be appreciated.


JB

 

[JB]

Something like one of these should do what you want.

http://www.xrite.com/documents/literature/en/L10-153_SP60_en.pdf
http://www.xrite.com/documents/literature/en/L10-157_962_en.pdf

Nice instruments, but a little like overkill for my needs I fear. Bookmarked
for the records though.
Thanks.

JB

 

[TKM]

In most lighting design software available today there are user-defined or
predefined surface reflectance values for many commonly used materials and
surface finishes found in building interiors.
In typical office lighting designs the 'default' values used in the UK
seem
to be 70:50:30 or 70:50:20 for Ceiling/Walls/Floor. This I suppose, is
meant
to represent typical matt or satin white paint for ceilings, white or
light
coloured paint on the walls, and some darker carpet for the floor.
In order to get a more accurate value for real reflectances of actual
surface finishes or materials, what instrument do you use to measure this
in
the 'real-world'?
I know I can use my spectrometer with a calibrated light source and
standard
certified reflectance target (like Labsphere Spectrolon etc.), but this is
not the sort of kit you want to be carrying around for casual
spot-measurements on site.
Are there any portable or handheld instruments on the market which can be
simply held against a surface to get a reading of total diffuse
reflectance?
If not, I'm thinking of building one for our own use, possibly based on a
group of RGB leds or even a tungsten lightsource and a detector such as
the
Taos TSL230 or TCS230.
Anyone use anything similar or have any ideas of 'homebrewed' instruments
they use in the field?
Any thoughts on this subject would be appreciated.


JB

There are two ways that I've used to determine such reflectance values.
Both are easy to do and yield reasonably-accurate results:

1 - There are charts of color and reflectance values. Some paint companies
have them or try looking at sites such as: http://www.cis.rit.edu/mcsl/
Simply hold the chart up against the unknown surface. Match the color and
brightness and read the reflectance value that applies.

2- A simple illuminance meter (calibrated in lux or footcandles) can be
used. Use a meter something like this one:
http://www.atlantalightbulbs.com/ecart/10Expand.asp?ProductCode=GE217

Arrange the sample of interest or take the measurements on a part of the
surface (such as a wall) so that is evenly lighted.

Hold the meter (or aim the cell of the meter) so that the light falling on
the surface of interest is measured. Now reverse the cell and move it a
hand's width or so away from the surface and measure the light being emitted
from the surface.

The per cent reflectance is the reflected light reading divided by the
incident light reading multiplied by 100.

These are not precision measurements, of course; but they are suitable for
interior lighting calculations.

Terry McGowan

 

[JB]

There are two ways that I've used to determine such reflectance values.
Both are easy to do and yield reasonably-accurate results:

1 - There are charts of color and reflectance values. Some paint
companies have them or try looking at sites such as:
http://www.cis.rit.edu/mcsl/ Simply hold the chart up against the unknown
surface. Match the color and brightness and read the reflectance value
that applies.

2- A simple illuminance meter (calibrated in lux or footcandles) can be
used. Use a meter something like this one:
http://www.atlantalightbulbs.com/ecart/10Expand.asp?ProductCode=GE217

Arrange the sample of interest or take the measurements on a part of the
surface (such as a wall) so that is evenly lighted.

Hold the meter (or aim the cell of the meter) so that the light falling on
the surface of interest is measured. Now reverse the cell and move it a
hand's width or so away from the surface and measure the light being
emitted from the surface.

The per cent reflectance is the reflected light reading divided by the
incident light reading multiplied by 100.

These are not precision measurements, of course; but they are suitable for
interior lighting calculations.

Thanks for the advice Terry. I did try a few readings like this using one of
my lightmeters albeit with mixed results. A lot of the painted surfaces in
our offices here have varying texture due to either lousy painting or rough
base material. I had to move the lightmeter away for the surface by about a
meter until I got satisfactory readings.
I started to build a reflectance meter tonight using a small Osram miniature
10W halogen reflector lamp aimed at ~45deg to the target surface, shielded
from a TSL230 light-frequency converter aimed at ~135deg. I just rigged up a
frequency meter to give me a relative output in kHz and 'calibrated' it
using a front surface mirror as a test target. Seems to work quite well,
especially with the darker carpets and painted surfaces. I'll now have to
build a constant current psu for the source and get a PIC micro interfaced
to an lcd display and stuff the hole lot into a case and the project is
sorted.
I'll post some pics when (if!) I do get it finished.
Now for some beers.....
Cheers,
JB

 

[TKM]

Thanks for the advice Terry. I did try a few readings like this using one
of my lightmeters albeit with mixed results. A lot of the painted surfaces
in our offices here have varying texture due to either lousy painting or
rough base material. I had to move the lightmeter away for the surface by
about a meter until I got satisfactory readings.

Sounds like you are on the right track. As you may have noticed when you
take the reflected light measurement, there is a point as you move the meter
away from the surface when the meter reading is fairly constant over some
distance. That's the measurement to use. It doesn't matter that the
surface is textured, what you are reading is a realistic value of the way
the surface is reflecting light.

I started to build a reflectance meter tonight using a small Osram
miniature 10W halogen reflector lamp aimed at ~45deg to the target
surface, shielded from a TSL230 light-frequency converter aimed at
~135deg. I just rigged up a frequency meter to give me a relative output
in kHz and 'calibrated' it using a front surface mirror as a test target.
Seems to work quite well, especially with the darker carpets and painted
surfaces. I'll now have to build a constant current psu for the source and
get a PIC micro interfaced to an lcd display and stuff the hole lot into a
case and the project is sorted.
I'll post some pics when (if!) I do get it finished.
Now for some beers.....
Cheers,
JB

You may have just invented a nice reflectometer.

Terry McGowan

 

[JB]

Be aware of uneven reflectance. That is some surfaces/textures reflect
differently depending on the angle of incidence. Polished wood or
brushed metal are common examples.

Yes this is true.
So you take readings at along the grain and at 90deg to the grain. Easy
enough.
This is less important of you have a well diffused larger area source and 2
or 3 detectors at angles to the incident illumination. I might include this
feature in the protoype.
JB