J. S. ompso 

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To: G. D. Mc Ghee 16 June 1969 

Subject: Flash Cleaning of Optical From: J. S. Thompson 

Surfaces in Vacuum 

The problem of cleaning optical surfaces in a vacuum, 

gravity-free environment has received some attention recently. 

Dr. James G. Baker suggested that a cleaning action may result 

from irradiation of the surfaces at high power and energy levels 

with an Edgerton flash lamp. 

Following this suggestion the Laboratories Division of 

the Aerospace Corporation set up an experiment to "calibrate" its 

feasibility. The attached pages are a report on their results. 

While their report is discouraging, their funds and time 

were limited, and they were unable to do more than examine the 

original, unadorned proposal. At these photon flux densities it 

seems quite probable that photoelectric emission must be occurring. 

If these electrons were swept away by zt:n• electric field it would 

leave a net positive charge on the contamination (assumed to be a 

dielectric). Raising the potential of the substrate at this point may 

aid in the removal of the contamination. 

While this may prove to be impracticable it is suggested, 

mainly, to indicate that one need not consider the issue closed. 

JST:im 

Attachment: Edgerton Flash Cleaning of Optical Surfaces; 2 pages 

cc: R. Belt 

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EDGERTON FLASH CLEANING OF OPTICAL SURFACES 

A helical xenon flash lamp was mounted in an aluminum block 

housing. The inside surface of the aluminum block was polished. 

An opening in the block provided a diffuse source of radiation. The 

energy input to the lamp could be controlled by the charge on the 

capacitor power supply; however, the maximum charge was used for 

all experiments. 

Sample reflective surfaces were mounted inside a vacuum 

chamber, and the radiant flux was introduced through a fused silica 

window. A transient thermopile was used to measure the incident 

energy. Absorbed energies were calculated from the incident energy, 

lamp spectrum, and the sample spectral reflectance. Radiant energy 

pulses were of the order of 0.001 second. 

The reflecting surfaces were coated with F-50 silicone oil, 

F-50 silicone oil with an absorbing dye, and F-50 silicone oil with a 

black pigment. Thicknesses of between 0.2 and 30 1-1were obtained. 

A free-standing polymer film was also irradiated. The use of a dye 

pigment increased the absorbed dose without changing the incident 

energy. 

For the pigment-free oils, absorbed doses of under 5 x 10+3 

joules/cc had no effect. Some oil renplyal for the thicker films was 

obtained with absorbed doses of 1 x 10 joules/cc. The efficiency 

decreased with the film thickness. 

_r4or the pigmented oils absorbed doses of approximately 

3 x 10 joules/cc resulted in partial removal; however, some decomposition of the oil was also produced which resulted in a carbonaceous 

layer on the surface. FoUhese carbonaceous deposits, even 

absorbed doses of 25 x 10 joules/cc had little effect. 

An analysis indicated that incident energies of 50 joules/cm-2 

would melt a thin aluminum layer. By !lacing an aluminized miror 

within the flash lamp in air, where approximately 100 joules /cm was 

obtained, the aluminum was removed. 

The critical parameters for the successful removal of surface 

contamination by a flash lamp appear to be: 

1. energy necessary for removal or evaporation of film, 

2. energy delivered to the film by optical means, 

-DORIAN 
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3. heat loss to the substrate, and 

4. damage threshold of mirror. 

The first factor depends on the nature of the film, i. e. , polymer, 

inorganic or carbon deposits. The second factor depends on the 

matching of the absorption spectrum of the deposit and the lamp's 

radiant spectrum and power level. The third distinguishes between 

thin and thick films. It is our opinion that this method will not serve 

as a general method of clearing contaminated surfaces. The method 

will become very inefficient for thin contamination layers and, 

because of chemical reactions, it may introduce more problems 

than it solves. 

- -DORIAN