VISIBLE LIGHT
Light, which is commonly referred as visible light, is a small, middle part of a large electromagnetic spectrum. On either side of this visible light are spectra that are not visible without special instrument.
The visible light is in fact polychromatic, i.e. it has seven colors of different wavelengths that are acronymed as VIBGYOR (Fig. 1.1). The white light will break into seven components when passed through a prism.
Wavelength of visible light extends between 400 and 700 nm = 4 × 10 12 to 7 × 1012 Hz (to be precise, 3.97–7.23 THz).
The wavelengths less than 400 nm are called ultraviolet and more than 800 nm are called infrared.
One nanometer is one-billionth of a meter, i.e. 10−9 m = 1/1,000,000,000 m.
Mixture of seven colors of spectrum will result in white light as in Newton's disk (Fig. 1.2).
By imposing a suitable interference filter between the source of light and a prism, the seven wavelengths can be separated in seven colors (Fig. 1.3). This phenomenon is used in production of laser.
Laser
The word laser is acronym for light amplification by stimulated emission of radiation.
Properties
- Laser is monochromatic.
- A particular laser has single wavelength.
- This depends on the medium used.
- It cannot be white.
- It is always colored, i.e. green, blue-green, etc.
- It is collimated, i.e. rays (photons) are exactly parallel.
- Polarization: The photons vibrate in the same plane.
- It produces bright light.
- It produces intense heat and energy at short distance.
- Laser can burn, coagulate, evaporate and disrupt.
- It can be concentrated in a very small area.
Construction
Laser consists of a cylinder that may be solid or hollow; latter is filled with gas, liquid or a combination. These substances should have ability to absorb energy in one form and emit a new type of more useful energy. The energy can be thermal, mechanical, light or electrical. The process of conversion is called lasing. A cavity of the cylinder has two concave mirrors at each end (Fig. 1.4). One of them is fully reflective. The mirrors are coated with thin film of dielectric that reflects light close to the wavelength of the laser light. The other mirror is located on the other end of the tube. The focal length of each mirror almost coincides with the center of the tube. The second mirror is partially reflective and is considered to be leaky. There are two slanting windows that close each end of the tube. The cavity or the rod is surrounded by source of energy that raises the energy level of the atoms within the cavity to a high level in a very unstable state. This is called population inversion. The next step is spontaneous decay of the energized atom to a lower energy level. This phenomenon is the basis behind the release of high energy in the form of light that is converted to suitable wavelength.
Figure 1.4: Optics of laser, M1 and M2 are two concave mirrors, focal length of which coincides with F, which is the center of the discharging tube. The concave mirrors are coated with dielectric film. M1 is fully-reflecting mirror and M2 is partiallyreflecting mirror. M2 is considered to be leaky. W1 and W2 are two slanting windows to close each end of the tube separately.
- Population inversion in active medium.
- Amplification of appropriate wavelength.
The energy stored in the laser material, i.e. gas, liquid or solid, is released in a narrow beam of monochromatic light. This light is a source of high thermal energy, which is used in ophthalmology for various purposes.
There are two modes of laser delivery:
- Continuous wave.
- Pulse.
The beam is focused on a small spot, which is measured in millijoules (mJ).
Each spot is exposed for milliseconds and power is measured in megawatts.
Various types of laser available are:
- Solid-state laser.
- Gas laser.
- Metal vapor laser.
- Excimer laser.
- Dye laser.
- Diode laser.
The uses of laser in ophthalmology are enlisted in Table 1.1.
Commonly used lasers in ophthalmology are given in Table 1.2.
Mode of Delivery of Laser in Ophthalmology
- Slit lamp
- Indirect ophthalmoscope
- Operating microscope
- Endoscope
- Surface probe.
Changes on a Surface When Light Strikes
When light strikes a surface, following things can happen:
- It is absorbed; hence the object looks gray or black.
- All the rays go back, hence the surface looks white.
- Only some waves are transmitted back; the object has a color of reflected ray only.
- If the surface is polished, the rays will be reflected.
- If the object is transparent, the rays will be refracted.
- When a beam of light is reflected, its color does not change.
- During refraction, the velocity of the light is hampered in denser medium.
- The speed of light in a given medium is less than in vacuum.
- Light can be polarized, partially polarized or remain non-polarized.
- The beam of light cannot be deflected by change in electric or magnetic field.
- Light can be made fluorescent by suitable exiting and barrier filters. This property is used in ophthalmology for fluorescein angiography.
Fluorescence is a phenomenon in which an object exposed to shorter wavelength emits light of longer wavelength. The character of reflected light is different from the original light. An object is said to be fluorescent when it has different colors, one by transmitted and other by reflected.
Outline of Fluorescein Angiography
The phenomenon of fluorescence is used to get a fluorescein angiograph of the retinal vessels. Fluorescein is used as aqueous solution of 2.5%, 5% and 10% as sodium fluorescein. Fluorescein has maximum absorption at 485 nm and peak fluorescence at 530 nm. To get a fluorescein photograph of the retinal vessels, 5 cm3 of 10% aqueous solution of fluorescein is injected in the antecubital vein in a bolus. It takes 8 seconds to reach fluorescein from arm to retina. In this, 60%–80% of fluorescein becomes serum bound, mostly albumin, remaining staying free. To get fluorescence, white light is allowed to reach the retina via a blue excitation filter. This allows only blue light (490 nm) to reach the retina. Fluorescein in blood vessel absorbs the blue light and emits a yellow-green light in 530 nm wavelength. This contains reflected blue light as well and returns to camera passing through a blue-blocking filter as yellow-green light, to reach the camera. Photographs are taken by a special fundus camera at an interval of 1 second between 5 and 25 seconds (Fig. 1.5).
| |||||||||||||||||||||||||
Fluorescein angiogram consists of:
- Prearterial phase (choroidal phase): This consists of fluorescein circulating in the choroidal circulation without reaching the retinal arteries.
- Arterial phase: This starts 1 second after the choroidal phase and lasts till all the retinal arterioles are filled.
- Arteriovenous phase:
- This is a transit phase.
- It consists of filling of retinal arterioles.
- The capillaries are also filled.
- The retinal veins show lamellar flow.
Measurement of Light
Light is measured by two methods:
- Radiometry.
- Photometry.
Radiometry measures total power of the light in watt. Photometry measures in candela, lumen, lux, foot-candle and apostilb. Lumen measures total amount of visible light emitted by a source of light. Lux (LX) is lumen per square meter.
Nomenclature of units for measurement of light:
- Candle and candela: Candle is the old term used to measure light in units based on the response to light. It has been replaced by a more precise term, candela.
- Luminance refers to the amount of reflected or emitted light by a cool surface. It is depicted by number of lumens per square meter incident on a given surface. A candela is equivalent to 4π lumens.
- Apostilb is defined as luminance of a surface that emits 1 lumen per square meter. The term apostilb is widely used in automated perimetry.
- The objects that cause luminance are called luminescent. There are two types of luminescent objects depending on duration of light emission after source of excitation is removed. They are fluorescents and phosphorescents. Light emitted from cool object is called luminescence. For example, fluorescein light, clock dials, glowing watches, television, etc.
GEOMETRICAL OPTICS
Properties of Light
- Light travels in a straight line.
- Its path can be reflected or deviated, but not bent.
- It comprises of many rays put together.
- A bunch of rays is called pencil of rays.
- The rays of light can be parallel to each other; they can diverge from a point or may converge to a point (Figs 1.6A to C).
- Divergence is referred to as negative convergence.
- Rays coming from infinity are considered parallel. They will not come to pinpoint focus unless passed through an optical system.
- For all practical purposes, rays coming from 6 m are considered to be parallel. Rays coming from a point less than 6 m are considered to be divergent. Shorter the distance; more is the divergence.
- When light strikes a surface, following things can happen:
- The light is completely absorbed and the object looks dark.
- Rays of some wavelength are reflected back. The reflected wavelengths impart color to the object.
- The rays are partially reflected back and partially pass through the medium. The medium is called translucent.
- If all the rays pass through the medium, the medium is called transparent.
- If the surface of the object is bright, almost all the rays go back, the phenomenon is called reflection.
- If the rays change direction, while passing through the medium, the phenomenon is known as refraction.
Fiberoptic Cable
Light travels in a straight line. It does not follow an angular or curved pat, but its direction can be changed in linear fashion with the help of multiple mirrors or prisms, or along the curve by fiberoptic cable also known as optical fiber.
This is a flexible, thin, cylindrical, dielectric waveguide that transmits light along its axis by using property of total internal reflection. A typical fiber consists of a core and a protective coat called cladding. Both are made of dielectric material. The cladding is surrounded by buffer, which in turn is surrounded by outer jacket. The refractive index of the core is more than the cladding. The light passing through the fiberoptic cable is not influenced by electromagnetic field (Figs 1.7A to E).
Effect of Light on the Eye
Interaction of energy of light with the retina creates neurochemical change that gives the sensation of sight. Before reaching the retina, the light passes through successive media of different density, each differing from the other.
Ocular Media
The various ocular media are:
- Tear film
- Cornea
- Aqueous humor
- Lens
- Vitreous humor.
Figures 1.7A to E: Optics of fiberoptic cables. A. Outline of fiberoptic cables; B. Light does not circumvent the bend; C. Light is reflected by 90° by mirror (not obstructed by bend); D and E. Light negotiating multiple bends through fiberoptic cables.
The various ocular surfaces that have optical effect on the eye are:
- Anterior and posterior surface of the cornea.
- Aqueous.
- Anterior and posterior surfaces of the lens.
- Anterior surface of the vitreous.
Absorption of Light in Different Media
The absorption of light in different media is variable:
- Rays with wavelength less than 295 nm are absorbed by cornea.
- Rays between 295 and 600 nm will pass through the cornea to reach the lens.
- Rays shorter than 350 nm are absorbed by lens.
- Rays between 350 nm and 600 nm will reach the retina. This is applicable only to phakic eyes.
- In the absence of lens, the rays between 295 nm and 600 nm will reach the retina.
- All the media of the eye put together are uniformly permeable to wavelength between 390 nm and 600 nm.
- Eyes with clear media are more sensitive to wavelength 550 nm that represent yellow-green spectrum.
- The eyes are less sensitive to ultraviolet and infrared rays.
- Rays beyond ultraviolet and infrared are invisible to normal eye, requiring special devices to be perceived.