El  dramatismo  en  la  voz:  los  ciclos  vocales

A rigid borescope is used as visual inspection aid and equipment to inspect internal surfaces such as bores of rifles, cannons, in- service defects in a variety of equipment such as turbines, automotive components and process piping etc. Similarly the borescopes are of immense use in aircraft and aerospace industry. Rigid borescopes are generally limited to applications with a straight - line path between the observer and area to be observed. Figure 3.7 represents a rigid borescope. Rigid borescopes range from 0.15 m to 30 m length and in diameters from 0.9 mm to 70 mm are available. The magnification is usually 3X to 4X, but powers up to 50X are available.

FIG.3.7. Rigid borescope

The illumination system is either an incandescent bulb located at the distal end or a light guide bundle made from optic fibres that conduct light from an external lighter source. For the illuminated surface the image is brought to the eyepiece by an optical train consisting of an objective lens, sometimes a prism, relay lenses and an eyepiece lens.

A brief description of essential components of rigid borescope is follows:

a) Light guide

The optical fibre bundle that carries light from an external high intensity source to illuminate the test part is called the light guide bundle. These fibres are normally of about 30 m in diameter; the size of the bundle is determined by the diameter of the scope.

b) Image guide

The image is brought to the eyepiece by an optical train consisting of objective lens, sometimes a prism, relay lenses and an eyepieces lens. The image is not a real image but in the air between the lenses. This implies that it is possible to provide both diopter correction for the observer and to control the objective focus with a single adjustment of the focusing ring at the eyepiece. The transmission of light in single optical fibre is limited and thus thousand of fibres are bundled for transmission of light and images. To prevent light from diffusing, each fibre consists of a central core of light quality optical glass coated with a thin layer of another glass with a different refractive index. This cladding acts as a mirror and all light entering the end of fibre is reflected internally and travels that cannot escape by passing through the sides to an adjacent fibre in the bundle. Some light is absorbed within the optical fibre depending on its length.

c) Focusing control

The rigid borescopes are of fixed focus type and focusing type. The focus control in rigid borescope expands the depth of field over non-focusing or fixed focus design.

d) Distal end (objective lens)

The choice of viewing heads vary according to their application, rigid borescopes generally have a 55 degree field of view although field of view can range from 10 to 90. Typically the distal tips are not changeable, but some models (extendable borescopes) may have interchangeable view heads.

e) Accessories

Many accessories such as still camera and video camera can be added to provide a permanent record of a visual test. Closed circuit television displays, with or without recording capabilities are common as well. Attachments at the eyepiece permitting dual viewing for increased accessibility are also available.

3.1.7.2. Flexible fibrescopes

Flexible fibrescopes are generally used in situations that do not have a direct line of sight to the observer such as around bends and corners. A typical flexible fibrescope is shown in the Figure 3.8.The flexible fibrescopes are available in diameters ranging from 1.4 mm to 13 mm and lengths up to12 m. Special quartz fibrescopes are available in lengths up to 30 m.

FIG.3.8. Flexible fiberscope.

The fibres used in the light guide bundle are generally 30 m in diameter. The fibre diameter in the image guide range from 6.5 m to 17 m for better image resolution. For enhancing image resolution further, an objective lens with a wider field of view and also to magnify the image at the eyepiece is used. The features of construction of a flexible fiberscope are discussed below:

a) Eyepiece lens

The eyepiece is a lens through which observer views the test area and image formed for visual inspection. The observer can attach a photographic or video camera to it using the appropriate adapters.

b) Diopter adjustment ring

It enables the inspector to adjust eyepiece to his vision by turning the ring till the image is in clear focus.

c) Focusing ring

For flexible fibrescopes with focusing type, the ring on the eyepiece adjusts the position of the objective lens in the distal tip through a connecting control wire. As the objective lens is moved back and forth, the instrument is focused.

d) Articulation control

Articulation knob controls two-way or four-way movement up, down, right and left of the distal tip. The distal tip contains the objective lens and the illumination window.

e) Light guide bundle

The function is similar to that in case of rigid borescope i.e. of transmitting light from the light source to the test area to be illuminated.

f) Insertion tube

It is also recognized as the working length or probe, contains the image guide and light guide bundles, and control wires. To one end of the light guide is light guide plug that is connected to light source to conduct the light to the test area. These bundles are encased in a special protective sheath consisting of flexible metallic spiral clad with plastic. The insertion tube is designed to be flexible and engineered to give a right balance between flexibility and stiffness.

3.1.7.3. Periscope

Periscope is an instrument used for remote observation of otherwise inaccessible areas. In simple periscope, two right angle prisms are utilised (as totally reflecting prisms) in combination with a series of lenses. Light entering and leaving each prism does not suffer refraction because the angle of incidence is zero. The periscope are of major use for remote visual inspection in hazardous situations such as radiation areas, toxic environment and for over head viewing of areas involving obstacles like walls and other opaque objects.

3.1.7.4. Videoscope

A video borescope has following four main components

1. A probe with a charge-coupled device embedded in the distal tip.

2. Video processor to communicate signals to the monitor.

3. A monitor, black and white or colour.

4. An alphanumeric keyboard for entering identification references into the display or into a permanent record.

These components are discussed below:

a) Video probe

Video probes are available in a variety of sizes, lengths and articulation options.

Similar to simple borescopes, video scopes test area is illuminated by fibreoptic light guide or by light from light-emitting diodes (LED). The lighting can be used for black and white or colour imaging. The illumination of the test surface is picked first by the fixed focus lens in the tip of the distal end and directs it to the surface of the charge-coupled device CCD.

b) Charge Couple Device (CCD)

The distal end of electronic video scopes contains a CCD chip, which consists of thousand of light-sensitive elements arranged in a pattern of rows and columns. The objective lens focuses the image of an object on the surface of the CCD chip, where the light is converted into electric signals. This is in proportion to the light falling on it. The signals travel down the length of the probe through a series of amplifiers and filters up to the video processor (CCU), of the Video image scope.

c) Video Processor or Camera Control Unit (CCU)

The image information of electric signals from pixels is digitized. The image can now be displayed on the monitor, recorded and if desired sent to a computer for enhancement and analysis.

Advantages of Video Borescopes:

1. As the image is viewed through a monitor, the fatigue & discomfort due to eyestrain and the operators’ positioning to see through the eyepiece in non-video borescopic are eliminated in video scopes.

2. The video borescopes allow multiple views of the same image making evaluation more reliable and facilitating training. The image can be transmitted simultaneously to any number of monitors at the site or to remote stations.

3. A video borescope has a large depth of field that is generally controlled by the CCU.

This saves time-consuming task of re-focussing as is in the non-video borescopes.

4. Magnification and resolution can be higher than non-video borescopes.

3.1.8. Lighting and light measurements