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RE: High Intensity Light Question
Tad - The 1 cd/c^2 statement indicates that your concern is with
retinal thermal injury and retinal photochemical (blue light) injury.
Both of these are related to the image of the source on the retina and
so this is an issue of direct viewing of the source(s).
Basically, if the radiance of the source (after applying the appropriate
spectral weighting) is sufficiently high, then a maximum permitted
exposure time (based on a Threshold Limit Value) exists. Although many
white light sources are of sufficiently high radiance to be a potential
hazard, they are not a hazard in practice because the high luminance
prevents fixation on the source due to discomfort and pain - as an
example, the sun at high elevations.
I used the word <radiance>, and it is the same concept as luminance
except the watt is used in place of the lumen (a visually evaluated
watt). If the light is white, there is a very rough relation between
radiance and luminance, sufficient for cd/cm^2 to be used as guidance in
whether a problem might exist.
Your particular problem depends only on the source properties (including
images of the sources due to reflectors) and the way in which they would
be viewed. It does not depend light as received by light meters.
One place to read about these UV/visible/IR hazard issues is in "Safety
with Lasers and Other Optical Hazards," Sliney and Wolbarsht, Plenum
Press, 1980.
The ACGIH TLV are presented in an excessively succinct manner. If you
are going to be considering such issues, I recommend that you obtain a
copy of ANSI/IESNA RP-27.1-96, Recommended Practice for Photobiological
Safety for Lamps and Lamp Systems -- General Requirements. This uses
the ACGIH TLVs but considers all of the details required when applying
the TLVs. A copy can be obtained from IESNA, 120 Wall St., 17th floor,
New York, NY 10005; call (212) 248-5000 for cost and ordering
information.
>----------
>From: Tad Blanchard[SMTP:Tad.M.Blanchard.1@gsfc.nasa.gov]
>Sent: Thursday, April 02, 1998 12:40 PM
>To: Multiple recipients of list
>Subject: RE: High Intensity Light Question
>
>Thanks Robert,
>
>A group of researchers are going to place 12-each 1kW quartz lamps on 2
>foot centers on a polished aluminum surface to simulate sunlight on an
>experiment.
>
>We wanted to insure that the workers and ancillary personnel will not be
>overexposed to the high intensity light as well as any associated UV during
>the use of this system.
>
>The ACGIH TLV book says that anything greater than 1 cd/cm^2 has to be
>evaluated since the TLV may be exceeded above this limit.
>
>Evidently we are not equipped to handle this type of evaluation since we
>only have a light meter that reads out in footcandles.
>
>Thanks for your help.
>
>
>
>
>
>At 11:17 AM 4/2/98 -0600, you wrote:
>>Tad -
>>
>><cd/cm^2> is a unit of luminance (formerly, photometric brightness) and
>>is a property of a ray [realistically, the average of a bundle of rays].
>> It is the differential luminous flux per unit projected area in the
>>direction of the ray and per unit solid. Thus this unit is its most
>>basic form is lumens per cm^2 and per steradian.
>>
>><footcandles> is a unit of illuminance, i.e., areal flux density. The
>>unit for <footcandles> is lumens per square foot (incident on an area).
>>
>>These units are not commensurate, and you cannot convert from one to the
>>other. I suspect that you have mixed up units in some manner or other.
>>If you had adequate information about the geometry of incoming luminance
>>at a point, you could then calculate the resultant illuminance at that
>>point.
>>
>>Robert E. Levin
>>Corporate Scientist
>>OSRAM SYLVANIA Development Inc.
>>71 Cherry Hill Drive
>>Beverly, MA 01915
>>(978) 750-1594
>>levin@osi.sylvania.com
>>
>>
>>
>>
>>
>>
>>>----------
>>>From: Tad Blanchard[SMTP:Tad.M.Blanchard.1@gsfc.nasa.gov]
>>>Sent: Thursday, April 02, 1998 10:30 AM
>>>To: Multiple recipients of list
>>>Subject: High Intensity Light Question
>>>
>>>Hi Radsafers,
>>>
>>>About 5 years ago I had to figure out how to convert candela/cm^2 to
>>>footcandles. I lost my reference data but seem to remember that
>>>one candela/cm^2 approximated 500 footcandles.
>>>
>>>Does anyone have the formula for this conversion?
>>>
>>>
>>>************************** /^\ /^\ ***********************************
>>>Tad Blanchard /__ \ /___\ NASA-Goddard Space Flight Center
>>>Nat'l Health Svc, Inc O Code 205.9, Greenbelt, MD 20771
>>>Sr Health Physics Tech / \ Phone: 301-286-9157
>>>Assistant RSO /___\ Fax: 301-286-1618
>>> mailto:Tad.M.Blanchard.1@GSFC.NASA.gov
>>> http://panza.gsfc.nasa.gov/205/205-2/Health/RADPROT.HTM
>>>
>>>************************************************************************
>>>
>>
>>
>************************** /^\ /^\ ***********************************
>Tad Blanchard /__ \ /___\ NASA-Goddard Space Flight Center
>Nat'l Health Svc, Inc O Code 205.9, Greenbelt, MD 20771
>Sr Health Physics Tech / \ Phone: 301-286-9157
>Assistant RSO /___\ Fax: 301-286-1618
> mailto:Tad.M.Blanchard.1@GSFC.NASA.gov
> http://panza.gsfc.nasa.gov/205/205-2/Health/RADPROT.HTM
>
>************************************************************************
>