The Tribune, Chandigarh, India

Glasses for eye safety
D.S. Chhabra

IN the modern age of industrial revolution, human beings are exposed to dangers from the industrial, chemical, mechanical and radiation environments at the work places or at their homes which adversely effects the body organs. The eyes, being the most sensitive and important organ, need special care to protect them from the risks of various types of radiations and mechanical impact hazards.

Before we go into the types of radiation hazards affecting our eyes, let us first know what is radiation. It is a form of energy which is transmitted, reflected, absorbed or emitted as particles or in wave form. It is also called electromagnetic radiation due to the presence of electrical and magnetic fields in the wave form. The band of radiations extends from alpha, beta particles, gamma rays and X-rays (all ionised radiations) to short wave Ultra-Violet (UV), long wave UV, Visible, short wave Infra-Red (IR), long wave IR, Microwave and Radio wave (all non-ionising radiations).

From the eye safety point of view we are especially concerned with short wave UV, long wave UV, excessive visible radiation and IR radiation as these cause eye cataracts, retinal damage, tissue heating and damage. The sun is the main natural source which emits these radiations whereas other man-made sources are furnaces; argon, excimer, He-Ne and CO2 lasers; halogen and xenon lamps; carbon arcs and welding radiations. Sun rays reflected from sea or snow etc are also some of the sources of short wave UV radiation. Harmful effects caused to the eye and its tissues will depend upon the intensity of radiation and the duration of exposure.

It is well known that excessive visible radiation is immediately sensed by the eye which helps in preventing the danger by closing the eyes or looking in another direction. However, eye is completely incapable of recognising the danger of invisible radiations i.e. UV and IR. Therefore, eye needs filter glasses which absorbs these harmful radiations and thereby provide protection.

The American Conference Of Governmental Industrial Hygienists (ACGIH) has published the Threshold Limit Values (TLV) for occupational exposures to UV, Visible and IR radiation incident upon eye. The TLV may be taken as a guide in providing the eye protection filter glasses to the workers to prevent the hazards of retinal, thermal and photo-chemical injury.

German standards — DIN 4646 and 4647 specifies the spectral transmission of eye protection filters required by the welder during gas and/or arc welding under various variants of work like size of electrodes, thickness of the work sample, and for hot welding etc. Depending upon the type of welding operations, the filter may have spectral transmission of 10% or less between UV wavelength region (320nm to 365 nm), 40% or less between Visible region (400nm to 700nm) and less than 30% for IR radiations (800nm-2000nm). These filter glasses are commercially available.

ACGIH also specifies TLV for permissible exposure to the unprotected eyes of radiation from various types of Lasers. Based upon the data, many foreign companies manufacture filter glasses for eye safety while working with Lasers.

Pilkington Special Glass Ltd., UK manufactures lenses which filters out hazardous UV and scattered blue light for use by the automobile drivers. But it is not known if it also filters out the IR. Central Scientific Instruments Organisation, Chandigarh, under a project funded by Department of Science & Technology, New Delhi has developed a filter glass device having a gradient coating which also filters out IR radiation along with UV and scattered blue light for use during night driving. The filter glass can be fitted on the wind screen of the automobiles in such a way that it can be flipped in and out of the driver’s line of sight as and when desired. The right hand portion of the gradient thickness of the coating attenuates the dazzle reaching the eye from the approaching automobiles and simultaneously permits clear view of the road from the left hand portion of the fiter glass. The filter glasses of all types should be tested by a spectrophotometric instrument to verify the required transmission characteristics.

Impact safety glasses:

All children, sports persons, car drivers, industrial workers and any other person whose occupation involves some degree of possible injury from the hazard of mechanical impact should consider eye-protection. The majority of the spectacle/goggles wearers are not aware of the highly brittle glass lenses in front of their eyes which upon impact would shatter into innumerable sharp splinters. Any accident which causes an impact to the lenses may lead to an eye injury which may be superficial or as serious as the loss of an eye. People should be aware that toughened safety lenses do exist to protect their eyes from the impact caused by the accident. But toughened lens is not unbreakable and should not be described as such. When a break does occur the glass crumbles into pieces which maybe rolled between finger and thumb without injuring. There are two types of toughening process-thermal and chemical.

Thermally toughened lenses are produced by sudden cooling of both surfaces of the lenses heated to about 650 degree Celsius by blowing forced air from the jets. The process produces a layer of compressed glass on the outer surfaces of the lens whereas glass in the centre develops a tensile thereby giving to the glass the required strength. The ophthalmic lenses having an edge or centre thickness on the order of 2.5 - 3 mm or greater are usually toughened by this process.

The alternate process of chemical toughening offers a higher degree of quality to the lenses as they are heated to only 470 degree celsius which is lower than the strain point of the glass. The process consist of immersing the lenses in a tank of molten salts of potassium to produce chemical equilibrium between the sodium ions in the glass and potassium ions in the molten bath. The ion exchange process displaces smaller sodium ions at the glass surface by larger potassium ions thereby creating high surface compression in the lenses. The process has the advantage of toughening much thinner lenses compared to thermally toughened process. Also, chemically toughened lenses are able to withstand approximately twice the drop-ball impact test compared to heat toughened lenses.

Toughened glass lenses are prone to breakage when subjected to an edge blow. Therefore, it is normally recommended that toughened lenses should preferably be mounted in a plastic frame rather then metal ones for extra safety, since a plastic mount would tend to absorb rather than transmit edge shocks.

The toughened lenses intended for industrial heavy duty use are tested as per international standard which specifies that the lens bears an impact from 32 mm diameter steel ball weighing 130 grams when dropped from a height of 1.83 metre onto it, whereas for light duty test 15 mm diameter steel ball is dropped from a height of 1 metre.

In order to meet global regulations on break resistance of glasses, there are manufacturers in the UK, Japan and Germany etc supplying ophthalmic lenses which are thermally or chemically toughened. These lenses also offer additional durability and scratch resistance.

The writer is ex-Senior Scientist & Head, Thin Film Coating Lab, CSIO, Chandigarh.

NEW PRODUCTS & DISCOVERIES
Walking on the wind

TEN years ago, Dutch scientist-turned-artist Theo Jonson had a vision: art that evolved. The evolution of his bizarre machines that walk when powered by gusts of wind took place on a computer.

Trained as a physicist (he was a doctoral student but did not finish), Jansen designed a programme that simulated pairs of legs of different lengths. He then created virtual creatures and raced them against each other to find the ones that moved most efficiently. These he built, and he hopes one day to find a way to let them evolve on their own.

In the meantime, he is working on ways to keep them moving even after the wind dies down. Their legs are comprised of pistons inside a tube, connected to a crankshaft. Once a gust of wind gets the sculpture going (most have polystyrene windmill blades or sails to give them a boost) and the pistons start moving, the legs could function as pumps and store compressed air. Popular Science

Robot-assisted therapy

Computerised "pets," such as those coming from Japanese electronics makers, could approach their flesh-and-blood counterparts in providing people with social interaction stimuli, scientists have said.

Purdue University is running a year-long study that puts an "AIBO" robot dog for six weeks in the homes of people 65 years and older who live alone, said Alan Beck, director of the Center for the Human-Animal Bond in Purdue’s School of Veterinary Medicine.

Cats and dogs have the well-documented ability to improve patients’ stress levels, blood pressure and other factors. Using robots could do the same while alleviating a medical staff’s worries about possible animal drawbacks, such as the need for feeding and exercise, Beck said.

"We want to see if robots can provide social stimulation in places such as assisted-living facilities and nursing homes," Beck told United Press International. "The robots have just enough movement and action to allow people to suspend their disbelief."

Using a robot also avoids the possibility of an animal being neglected or abandoned, Beck said.

Although people have been suspicious of the idea at first, positive reactions have come quickly and strongly, Beck said. The first participant, in her 90s, started off thinking the robot’s needs would include things such as batteries. UPI

SCIENCE & TECHNOLOGY CROSSWORD

Clues

Across

1. Lines drawn on maps connecting poles and all places having mid-day at same time.

8. ….. intensity tells the strength of an electric field at a point.

10. A technical aid program for the handicapped providing high tech products to them. (abbr.)

11. Coloured part of the eye of vertebrates.

13. A top observatory based in Munich.

14. Symbol for Cadmium.

15. Brain of the computer.

17. ….wort, a brown flowered plant used in herbs.

18. Symbol for Copper.

19. This institute carries research and training related to reproductive technology. (abbr.)

20. The gas used in vehicles.

22. It implies ‘divided’ by the amount of substance.

24. An instrument to grip a thing to work upon in its jaws.

25. ….mine is a white crystalline substance forming a thermosetting resin with formaldehyde.

26. Now commonly used in heart surgery.

27. A function of a variable quantity x approaching a limiting value k as x approaches a value a.

Down

1. A solid body from outer space that becomes a shooting star on entering earth’s atmosphere.

2. Property of a body to resume its original form on removal of forces acting upon it.

3. The oscillations of electrons between atoms within an ion or molecule.

4. A condition caused by consuming excessive alcohol, disturbing nervous system.

5. A rare metal resembling platinum and extremely hard.

6. A mixture of gases.

7. A computerized research centre having information of 5 million criminals & answering 2 lac queries everyday. (abbr.)

9. Symbol for Potassium.

12. The galaxy to which solar system belongs has this form.

14. A group of synthetic organic compounds harming the environment and banned now.

16. A region of complete shadow.

19. Third and last portion of small intestine.

21. A unit of an organism controlling individual inherited characteristics.

23. ….vorous are ones feeding on both plants and flesh.

Solution to last week’s Crossword: