Due to the thermoelectric effect, the temperature difference causes an electrical voltage to build up within each thermocouple. This output voltage is directly proportional to the power of the incoming radiation.[10] Since a large number of thermopiles are typically connected in series, voltages of several μV to V are reached.

Spectacle lens in the frame – The inside rim of the spectacle frame is traced over the Fresnel prism. This is then cut with the help of scissors and can be applied over the spectacle lens.[26]

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As shown in Fig 2, a thermopile laser sensor consists of several thermocouples connected in series with one junction type (hot junction at temperature T1) being exposed to an absorption area and the other junction type (cold junction at temperature T2) being exposed to a heat sink. When a laser beam hits the surface of a thermopile sensor, the incident radiation is absorbed within the coating layer and transformed into heat. This heat then induces a temperature gradient across the sensor given as

Some of these patients might have underlying uncontrolled diabetes or hypertension, which needs urgent intervention and be life-threatening to the patient. Interprofessional coordination between physicians, orthoptists, nurses, and ophthalmologists, including squint or neuro-ophthalmologists, ensures better management and helps the patient make better decisions. Once underlying causes have been ruled out, dispensing Fresnel prisms requires coordination between strabismologist and orthoptists.[21]

There are no clear contraindications in prescribing Fresnel prisms. But there are a few precautions that should be kept in mind before prescribing Fresnel prisms.

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In this method of cooling the heat is actively transferred to the environment. This is usually done by mounting a fan on the heat sink of a passively cooled detector or by pumping water through a channel system to cool the sensor. The preferred choice depends on the amount of heat to be dissipated and thus on the detector power.

Manufacturing processes require precision and reproducibility. For laser materials processing the monitoring of laser power is beneficial as it can avoid scrap production and yield high quality products.

Flanders et al. shared their clinical experience among 141 patients prescribed Fresnel prisms. They reported that 90% of 127 patients received Fresnel prisms over the non-dominant eye. Of the patients, 80% had a successful outcome, with relief of double vision. There was a dropout of 6%; eight patients discontinued prism because of decreased vision with prisms, persistent double vision, torsion, or optical aberrations.[29]

Initially, they were intended for use in lighthouse beacons. Prisms have been used extensively in the field of ophthalmology, especially related to neuro-ophthalmology and strabismus.[3] Prisms are used extensively to increase the binocular field, relieve diplopia, or expand the field of vision. Fresnel prisms are thin transparent plastic membranes with multiple tiny prisms attached from base to base. Augustin Fresnel articulated the optics of Fresnel prisms.[4]

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The maximum power that can be measured accurately depends on the type of sensor, its material properties and the type of cooling used (see section 1.3).[12] Faulty measurements or even deterioration of the sensor can result due to too large irradiance.[12]

The nursing, allied health staff, and interprofessional team help in recruitment, evaluation, prism prescription, regular counseling, and follow-up of patients requiring Fresnel prisms.[30]

The prism is stuck to the spectacle lens with the help of water. If any air bubble gets entrapped, the air bubble enters, and the prism separates from the spectacle lens. Another entity is the hemianopic Fresnel prism which is used to treat patients with hemianopic visual field defects by putting a 30-diopter Fresnel plastic prism which is pasted on the glasses.[2]

The spectral range depends on the absorption characteristics of the coating material.[17] Typically, a flat absorption spectrum across a broad wavelength range is desired. It can also be tailored to a wavelength range or to a particular wavelength.

The background error can be minimized by keeping the sensor at ambient temperature and avoiding convective air flows. It can also be corrected by subtracting the signal of a non-illuminated sensor (dark measurement).[19]

An arrangement of several thermally coupled thermopile sensors similar to a quadrant photodiode design (Fig 9) can be used to detect beam position as well as beam power. This is useful for beam alignment purposes or for processes where a correct beam position is crucial for high production yield.[21]

The Fresnel prisms can be prescribed permanently or temporarily based on the underlying indications. The most common personnel involved are either orthoptists or ophthalmologists.[21] Optometrists can also prescribe Fresnel prisms after due consultation with an ophthalmologist. The formula for prescribing Fresnel prisms is essential while prescribing these prisms. Prism dioptres of Fresnel prism = 2/3 (phoria) – 1/3 (compensating fusional vergence). If the patient has an exophoria of 9D and a base out prism of 6D is needed to blur, the prism power required would be 2/3(9) – 1/3(6). Thus, this patient would need four D dioptres base in prism to avoid diplopia in exophoria patients.[8]

The flexible prism sheet is thinner than 1mm in thickness. The prism is cut in the spectacle glass's shape and stuck to it.[24] A prism adaptation test should be done before prescribing Fresnel prism. The prism is cut such that it is smaller than the spectacle lens, which is about 1mm inside the spectacle edge. It is essential to take care that the prism membrane should not overlap the spectacle lens edge or the frame, as this may allow air bubbles to be trapped between the prism and the spectacle lens.[25]

Fresnel prisms are used to relieve diplopia. These can be used in fourth and sixth nerve palsies, restrictive motility secondary to thyroid-related orbitopathy, and convergence insufficiency. Only some patients better accept these in comparison to conventional prisms. These are also used in patients temporarily before surgery. This helps confirm the deviation in prism dioptres and better surgical planning. After surgery, Fresnel prisms play an essential role in avoiding postoperative diplopia.[29]

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The maximum laser power density for the sensor is given by the laser induced damage threshold of the coating material.[13] The threshold value depends on the wavelength of the laser, its pulse length and to a certain extent, on the structure of the absorbing surface [13]

In this method of cooling the cold side of the sensor is mounted onto a heat conductor (usually an aluminium heat sink), and heat is dissipated to the surrounding by conduction (through heat conductor) and convection (air flow).[15]

Attachment of Fresnel prism sheet onto the spectacle lens. First, the spectacles and the Fresnel prisms are washed with a gentle liquid detergent to clean grease or soil. Then submerge the spectacles and Fresnel prism in a large cup full of lukewarm water. Clean small air bubbles clinging to the surface. Position the Fresnel prism with a smooth side towards the inner layer of the spectacle lens with water. The final adjustments can be made by pushing the Fresnel prism and pressing it dry. The edges of the Fresnel prisms should be inspected to rule out any overlapping over the spectacle frames. Remove and reapply if any dust particles or air bubbles are noticed.[27]

The Fresnel prism trial sets are an indispensable tool for evaluating ocular motility. These prisms allow for the accurate evaluation of large angle deviations. The Fresnel prisms are available as rigid and loose stick-on prisms.[21] The flexible prism membrane is made from optical-grade polyvinyl chloride (PVC). Any patient presenting with a squint or double vision needs to be thoroughly evaluated. The ophthalmic instruments required would include a loose Fresnel prism, binocular single vision charts, diplopia charting, Hess charting, and Goldman perimetry for testing the binocular field.[8]

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Fig 3(b) shows the cross sectional view of the axial sensor where the temperature difference is established between the top and bottom surfaces. Thermocouples are embedded into a matrix and aligned parallel with respect to the heat flow, forming junctions at top and bottom.[8] This arrangement permits a reduction of the total sensor thickness to 0.5 mm (Fig 4).[8]

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Preparing Fresnel prisms is done by placing the Fresnel prism lens in the desired direction on the carrier lens. The Fresnel prism is cut nearest to the size of the carrier lens. Final cuttings are done to match the Fresnel lens to the carrier lens in size and shape.[22]

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The sensitivity S [V/W] is the ratio of voltage U [V] generated due to the incident laser power P [W] on the sensor. The voltage generated depends on the Seebeck coefficient of the thermoelectric material; hence it is a material specific constant.[9] The incident power can be calculated by measuring the sensor voltage and using the formula:

For sporadic measurements outside the laser system (e.g. during maintenance) a separate measuring unit is beneficial. For such a power meter, the sensor element is usually integrated into a metal housing for mechanical and thermal stability. The signal is recorded and processed in a read-out unit which displays the measured laser power (Fig 8).[21]

A prism is a wedge-shaped glass piece made up of transparent material. They have the property of a light ray's deflection towards the base. Prisms are routinely used in orthoptic practice to assess the degree of squint and manage deviations so that the eye becomes orthotropic. Prisms have been used in ophthalmology to treat binocular pathologies for over 100 years. However, only after the 1950s did instrumental orthoptics not provide the desired results; pediatric ophthalmologists started using prismotherapy as a prime treatment modality. As new prisms became available, the old conventional ones with high powers were aborted. After 1960, the Fresnel principle, which existed for more than 150 years, was used for the first time to manage strabismus. Fresnel prisms are made up of plastic sheet, which is thin and transparent, and it induces a prismatic effect. One side of the prism has angular grooves, and the other smooth side is attached to the lenses of the glass. This activity will describe the anatomy and physiology, indications, contraindications, equipment, preparation, technique, complications, and clinical significance of Fresnel prisms.

After 1960, the Fresnel principle, which existed for more than 150 years, was used for the first time to manage strabismus. In 1970, the optical scientific group from San Rafael of California designed a combination of a series of Fresnel press on prisms.[1] These prisms are made up of plastic membranes with powers ranging from 0.5-30 prism dioptres. The Fresnel prism is derived from the concept of hand-ground lenses prepared by the physics and French specialist Augustine Fresnel in 1921.[2]

There are two main types of thermopile laser sensors which can be classified according to the geometric arrangement of the thermocouples inside the sensor element.

As an alternative, detectors can be used for sporadic measurements at the laser system output. Usually, the full beam is measured in this case.[21]

Fresnel prisms are most commonly used as stuck-on prisms. Traditionally, prisms have also been used in patients with stroke and homonymous hemianopia. The aim is to shift the peripheral image towards the central retinal meridian.[7]

Unlike photodiodes, thermopile sensors can be used for a broad spectrum of wavelengths ranging from UV to MIR (depending on the characteristics of the absorption coating at different wavelengths).[4][5] Further, photodiodes are reverse biased and saturate for optical powers above a certain value (typically in mW),[6] making thermopile sensors suitable for high power measurements.[2]

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Deterioration of visual acuity due to reflections is common with prisms greater than ten prism dioptres. The vision deterioration with Fresnel prisms is more than with conventional prisms. A 30-prism dioptre conventional prism might reduce visual acuity from 20/20 to 20/30. This is lesser than the Fresnel prism, which reduces visual acuity from 20/20 to 20/100.[8]

Objectives: Describe the optics and advantages of the Fresnel prism.Explain the process and technique of Fresnel prisms application.Outline the clinical uses of Fresnel prisms.Summarise the contraindications and clinical problems associated with Fresnel prisms. Access free multiple choice questions on this topic.

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In general, a thermopile sensor consists of three elements: an absorber, the sensor element and a cooling body to dissipate the incoming heat.

Thermopile laser sensors find their use mainly where sensitivity to a wide spectral range is needed or where high laser powers need to be measured. Thermopile sensors are integrated into laser systems and laser sources and are used for sporadic as well as continuous monitoring of laser power, e.g. in feedback control loops. Some of the applications are

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Thermopile laser sensors (Fig 1) are used for measuring laser power from a few μW to several W (see section 2.4).[2] The incoming radiation of the laser is converted into heat energy at the surface.[3] This heat input produces a temperature gradient across the sensor. Making use of the thermoelectric effect a voltage is generated by this temperature gradient. Since the voltage is directly proportional to the incoming radiation, it can be directly related to the irradiation power (see section 2.1).

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It is essential to check the optical center of the Fresnel lens with that of the carrier lens. The smooth surface of the Fresnel prism is placed facing the inside surface of the spectacles. These are available from powers ranging from 0.5 prism dioptres to 30 prism dioptres.[23]

The signal rise time is the time required by the sensor to reach 95 percent of the full signal amplitude when exposed to a step function of incident laser power. It depends on the overall thermal resistances and thermal capacitance of the sensor.[11] The magnitude of these two parameters depends on the detector materials and geometry [11] The rise time for axial sensors is usually shorter than for radial sensors since the axial sensors possess lower thermal mass and thermal resistance.[8] The difference can amount to a factor of 5 to 10 and is shown in Fig 5.[8]

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To protect the sensor from damages by short optical pulses, volume absorbers are used with absorption lengths in the order of millimetres.[11] This enables volume absorbers to withstand higher pulse energy densities, since the optical power is absorbed over a considerable depth of material.[11]

Thermopile discs have thermocouples deposited onto an aluminium plate in a radial arrangement as shown in Fig 3(a).[8] All thermocouples are electrically connected in series with one junction at the circumference of the inner area which is illuminated and the other junction at the outer circumference.[8] The absorption coating in the illuminated area converts radiation into heat which flows radially outwards generating a temperature gradient between inner and outer ring and thus a thermoelectric voltage.[8]

Patients who complain of double vision need a detailed squint evaluation. Clear history regarding onset, duration, progression, diurnal variation, and prior episode of trauma/ fever/ association with underlying systemic disease needs to be explored. An interprofessional collaborative approach involving orthoptists, optometrists, ophthalmologists, and physicians is required. A strabismologist or neuro-ophthalmologist opinion should be sought whenever needed.[29]

Short-pulsed lasers which are used in spectroscopy and optical communication can be measured using thermopile sensors since they possess high thresholds for laser induced damages, especially when equipped with a volume absorber. (see section 2.5).

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According to EU standard (EN6001-1-22), every medical laser system needs to be equipped with a redundant power measurement unit. For procedures such as precise tissue cutting and ablation the laser power can be measured before operation or even continuously throughout the process. One possible means of integrating a thermopile sensor in a medical system is by using a shutter or beam reflector (Fig 6) which can be flipped into and out of the beam path for short measurement periods of the full laser power.[21]

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Spectacle lens out of frame – The Fresnel prism is cut with the scissors as close as possible to the size of the carrier lens. The optical flush is trimmed along the beveled edge of the spectacle lens with the help of a razor blade. This is then inserted into the spectacle frame.[2]

If the sensor temperature is different from the ambient temperature heat flows directly to the surrounding without contributing to the detected temperature gradient therefore effectively reducing the sensor output.[20] This type of error is on the order of few mW and is thus significant only at low incident powers[20]

Pyroelectric sensor and calorimeter are commonly used for measuring the energy of laser pulses.[7] Pyroelectric sensor can measure low to medium energies (mJ to J) and are prone to microphonic effects.[7] Calorimeters are capable of measuring high energies (mJ to kJ) but have large response times.[7]

Since the dependence is quasi linear, the temperature error can be corrected by multiplying the measured value by a temperature dependent correction factor[19]

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The sensitivity of the sensor varies with the mean sensor temperature. This is due to the temperature dependence of the Seebeck coefficient (see section 2.1).[18]

Fresnel prisms are made from a thin transparent plastic sheet consisting of multiple angular grooves on one side. These prisms are much lighter than the regularly used prisms of the same power.[8] It can be thought of as a series of tiny plastic prisms placed on a plastic platform that is thin and adjacent to each other in a format that the base of one is attached to the apex of the next prism. The magnification effect seen with conventional prisms is minimal with Fresnel prisms.[9]

For surface absorbers the thickness of the absorption layer is very thin (0.1 – 100 μm) and so is the total absorption length.[11] It is used for power measurements of lasers with long pulse length (generally for CW laser). If a laser with pulse length in the range of 10−7 – 10−4 sec is used the sensor can be damaged by either dielectric break-down or thermal effects.[12] In case of thermal damage, heat is deposited in a short time and cannot be dissipated until the next pulse arrives. This leads to an accumulation of energy in a thin layer leading to partial vaporization.[11] For dielectric breakdown, the peak energy density during a pulse is high enough to locally ionize the sensor surface.[13]

The nursing, allied health staff, and interprofessional team help monitor these patients to determine whether they are improving with Fresnel prisms.[31]

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The effective sensitivity depends on the absorption property of the coating layer. For constant incident laser power a larger absorption coefficient means more heat is generated[16] leading to increase in output voltage.

There are various ways of integrating a power measurement. In Fig 6 the integration in the beam path behind a beam splitter is shown. Fig 7 illustrates the option of mounting the detector behind the back mirror of a laser cavity for continuous monitoring. Beam losses further down the beam path, caused e.g. by a deterioration of optics, are not mapped in this type of arrangement.

It is crucial to dissipate the incoming heat in order to establish a stable temperature gradient across the sensor.[15] Therefore, the cold side of the sensor needs to be thermally coupled to a heat sink.

The angle of refraction of a prism depends upon the angle of the surface and the refractive index of the material of which the prism is made.[5] The angle of refraction is independent of the thickness of the prism. These offer the advantage of being lightweight and easy to handle. The most common use is to temporarily relieve the diplopia in cases like nerve palsies secondary to trauma, blowout fractures, decompensated phoria, divergence insufficiency, or convergence insufficiency.[6]