Laser pointers

J. Claire Dean <clairedean<-a t->aol< . >com> writes

>Does anyone have references or thoughts with regard to the possible
>longterm impacts of regularly using laser pointers (the small hand
>held type used to indicate points of interest on a screen when
>giving a slide presentation) to highlight areas of interest on works
>of art?

I addressed the question to a friend of mine Dr. Igor Turovets
<turovets<-a t->Hadassah< . >org< . >il> who is a specialist in laser technology
and I preferred to present his complete answer except some really
complicated equations, otherwise, the information is very important
I believe.

    We consider the laser primarily as a source of thermal energy
    and assume that a beam of laser irradiation of constant
    irradiance 'I' is incident normally onto a smooth material
    surface. To calculate the heat flow 'H' we assume (1) that there
    is no melting, (2)the absorption coefficient 'a' (alfa) is
    constant and a laser pulse has a rectangular shape with the
    pulse duration 'g' (gama). The heat flow 'H' at the surface is:

        H = I x (1-R)

    where 'I' is the irradiance and 'R' is the surface reflection.

    Laser pointer are generally diode lasers that produce radiation
    at wavelength region 650-675 nm, red light. There are many
    different pointers, but generally speaking they have 3-5 mW
    output and few milliradian divergence angles. The minimal point
    size at 52' found in prospects of different pointers distributed
    by Edmund Scientific (1996 Catalog) is 4 mm * 0.6 mm. Thus the
    irradiance of the pointers is

        I < 5 mmW (4mm * 0.6mm ) = 2000W/m2.

    We can calculate the maximal surface temperature and it will be
    an overestimation, because the calculated temperature represents
    the case of infinitive surface irradiated with infinite beam.
    The overestimation will be even greater if we assume the zero
    reflectivity of the sample.

    With the above mentioned assumptions we will make calculation
    for two cases to present materials with large and small thermal
    conductivity (and diffusivity). For example we will take Ag (A)
    and Ag2S (B). The temperature rise after 1 minute of irradiation
    will be < 0.5 and < 10 Celsius in case A and B respectively.
    Such a small temperature rise (overestimation!) shows of
    possibility for thermal damage caused by pointers to art
    objects. Photochemical damage could be in principle caused by
    the laser beam, but both (1) relatively small intensity of the
    pointers and (2) wavelength region where the photochemical
    alterations are known to be extremely rare, make the probability
    of the photochemical damage negligible for all objects, except
    the eye.

Thanks to Dr. Igor Turovets.

Michael Maggen
Senior Paper Conservator
The Israel Museum. Jerusalem,Israel

                                  ***
                  Conservation DistList Instance 10:62
                 Distributed: Friday, January 10, 1997
                       Message Id: cdl-10-62-001
                                  ***