Fundamentals of Projections

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fundamentals of projections

According to your messages about how projections work or fundamentals of projections, I dediced to compile this post to help you understading the basics of projections and the projection screens. Hope you will enjoy the post. Thx, for all your frank interest.


Fundamentals of Projections

Screen Sizes:

Most of the projection screens are custom manufactured to the customer‘s specification. High-frequency welding process allows the production of screen surfaces in almost any shape and size. There is no limit on the width of a screen, but there are practical and physical limitations on height. For screens that are higher than they are wide, we generally recommend reinforced screen materials to prevent “hour-glassing” on the sides and sagging in the middle. Unfortunately, reinforced screen material is only available for front projection screens. The overall weight of the screen should be considered when planning or purchasing a custom size.

projection screen size

Screen Production:

Most of the manufacturers employ a state-of-the-art high-frequency welding process to manufacture top-quality, professional projection screens. This process encompasses several steps which produce nearly invisible seams. This method also allows the production of high-gloss vinyl film and other PVC screens. Inflatable screens with air valves are also available.

 

embossing of projection screens

Embossing:

Most of projection screens have a slight embossing on one side that helps direct the light that is projected on or transmitted through the screen. For both front and rear projection, the embossing should always face the viewer/audience. (The only exception: HIGH GAIN screens, for which the embossing should be on the back so that the smooth, highly reflective side faces the viewer/audience.)

 

PVC / Thermoplastic:

Many screens are made of PVC (polyvinyl chloride), an amorphous thermoplastic that is white, hard, and brittle until it is treated with plasticizers and stabilizers that make it a malleable material usable for technical applications. Additional additives make the material flame retardant.

 

temperature effect on projection

The “Temperature Effect” :

The physical properties of PVC change with temperature, so PVC screens become more supple under heat and harden when they become cold. Therefore, unless projection screens are handled, installed, and loaded at ambient room temperature, the “Temperature Effect” can occur, which can cause irreversible damage to the material.

 

crazing on projections screensCrazing:

Many amorphous plastics, including PVC, form small white streaks when stretched, folded, or otherwise stressed. This phenomenon is called crazing and creates tiny irreversible flaws that, depending upon the severity of the damage, can expand into visible cracks in a projection screen.

 

Finish:

Model 0: Vertically welded panels, no edge finishing.screen manufacturing models

Model 1: Reinforced (65 mm / 2.55″) PVC webbing with grommets (inside diameter 12 mm / 0.47″) spaced 200 mm / 8″ on center at the top, small (40 mm / 1.57″) welded hem on the sides and a 100 mm / 4″ pocket on the bottom.

Model 2: Reinforced (65 mm / 2.55″) PVC webbing with grommets (inside diameter 12 mm / 0.47″) spaced 200 mm / 8″ on center on the top and sides, 100 mm / 4″ pocket on the bottom.

Model 3: Reinforced (65 mm / 2.55″) PVC webbing with grommets (inside diameter 12 mm / 0.47″) spaced 200 mm / 8″ on center on all sides.

Model 4: Reinforced (65 mm / 2.55″) PVC webbing with snaps spaced 200 mm / 8″ on center on all sides.

Skirt: Finish as model 1 or 2 with skirt on front side of screen at bottom edge.

 

truss frameTruss Frame:

Aluminium truss Size: 290 x 290 mm / 12″ x 12″ Quick and reliable frame assembly with plug-in type connectors. Tensioning of screen (with eyelets): Bungee cord fastener.

Spannfix Bungee Cord Length approx: 270 mm / 11″

 

Contrast:

Contrast is simply the ratio between black and white.contrast Picture contrast is considered good when black areas are perceived as highly distinct from white areas. To determine the contrast ratio of a projection screen, we use a checkerboard grid of eight black and eight white squares and an NIT meter that calculates the average values of the black and white squares. For projection under normal conference room conditions, the following is a contrast guideline: a ratio from 6:1 to 10:1 is considered bad; 20:1 and up is considered good.

 

greyscreen projectio screenBlack Level:

The black level is the intensity of the color black when projected onto a projection surface. The black level is influenced by the projection source and the projection screen. Optimal black level would assume a theoretical 0% reflection and transmission of the incident light in the black image areas. 1 Text projection of scattered light on a white projection screen. 2 Text projection of scattered light on a GREYSCREEN projection screen.

 

Image Quality:

The quality of a projected image is dependent not only upon the screen material but many other factors, including projector brightness, ambient light, reflected light and distance from the projector to the projection screen. Projection in daylight situations is possible only under very specific and controlled conditions.

 

luminance factor (gain)Luminance Factor (Gain):

The luminance factor, also called gain, describes the efficiency of the screen and its ability to gather light. The criteria for establishing the gain of a projection screen as compared to a defined reference material (standard white DIN 50339) is described in DIN 19045. This reference material is made from barium sulphate, a white chalk tile that has a standard gain of 1.0. When a measurement has a gain greater than 1.0, it reflects (front projection) or transmits (rear projection) more light than barium sulphate. Gain is measured at different angles and is greatest at a 0° axis, when both the projected light and the viewer are parallel to the viewing surface. Gain decreases as the viewing angle becomes wider.

 

Scatter Angle and Half-Gain Angle:

The scatter angle of a projection screen indicates the outside limit of the viewing cone per DIN 19045. This limit is defined as 40° left and right of the 0° projection axis. The half gain angle refers to the limit at which the gain falls to 50% of the 0° center gain. Half gain is not usually mentioned as an additional criteria for measuring the effectiveness of a projection screen.

 

Luminance Diagram:

The luminance diagram shows the luminance factor at each particular viewing angle. A flat curve progression indicates a uniform luminance distribution on the screen. A high rising curve indicates that the screen may have a tendency to hot-spot and may not be suitable for certain applications such as soft-edge blending projections.

 

ANSI-Lumen:

ANSI-Lumen is the measurement of the luminosity of a projector. It is used to compare projector performance. Luminosity is determined by the intensity of light (in lux) and the size of the illuminated projection surface (in m²). (ANSI = American National Standards Institute, the U.S. authority for standardization of industrial procedures. Its counterpart is the German Institute for Standardization, or DIN. Both are members of the International Organization for Standardization or ISO).

 

Picture Format:

The picture format is the width-toheight ratio of a defined picture. • Square Format 1:1 • Panorama Format 2:1/3:1 • Slide-Format 3:2 • Video Format 4:3 • Cinema or HDTV Format 16:9 • WUXGA Format 16:10 (for Full-HD Projection with 1920 x 1080 Pixels) When planning for a projection screen, it is useful to know what format will be used most often.

 

Screen Size according to DIN 19045:

As a rule of thumb, the width of the screen should be more than 1/6th of the maximum viewing distance and the minimum distance to the screen should not be less than 1.5 times the screen‘s width. The height of the screen is determined by the preferred format (e.g. 4:3 or 16:9).

 

Screen Types D and R:

Screen type D (diffuse) describes a projection screen in which light is cast and reflected, and the reflected light is not controlled. Screen type R (rear projection) does not reflect the light that is cast on it, but rather transmits light through its surface from behind. Type R controls the percentage of light that passes through the surface, but not the direction in which the light is transmitted. Projection screens categorized as D/R reflect and transmit light simultaneously. Type B projection screens have a crystal glass coated surface which helps focus light, as do Type S projection screens which have a silver metallic coated surface.

 

Reflection, Transmission and Absorption:

Degree of reflection indicates the ratio of reflected spectral flux to the projected spectral flux. Degree of transmission represents the ratio of penetrated radiation flux to absorbed spectral radiation flux. The value of 1 (100%) represented in the color spectrogram consists of the reflection, transmission and absorption of the projection screen. Loss of spectral radiation through absorption is very minimal. Reflection factor and transmission factor are often mistaken for gain, but the method and significance of the measurement is fundamentally different.

 

Colour Spectrogram and Colour Reproduction:

The colour spectrogram shows the reflection or transmission of the visible spectrum (approx. 380 – 780 nm) over the entire screen surface. The reflection and transmission of visible light on a projection surface can vary significantly across the entire colour range. Compared to the luminance factor, the colour spectrogram indicates the average value, in nanometers, of the entire visible spectral range over the entire screen surface. Low reflection or transmission values are not necessarily associated with a low luminance factor. The actual colour reproduction of a projection screen has less to do with the degree or the intensity of reflection and/or transmission and more to do with the evenness of the various wavelengths. The more even the measured values are, the more even the colour spectrogram will appear and the better the actual colour reproduction of the screen will be.

 

Soft-Edge Projection:

With soft-edge blending, media software is used to control the intensity of projected light in the overlapping portion of a picture being projected from two or more projectors. This ensures a smooth seamless image. Media software can control only the projectors, not the surface. The overall success of any system depends as much upon the projection surface as it does on the media software. Variables such as the luminance factor (gain) and viewing angle are critical to smooth, blended images. Soft-edge projections work best when the screen delivers uniform luminance across the entire viewing angle. Screens with a high gain and unfavorable luminance distribution will result in darker, uneven areas at the image overlap. Both creamy-white and grey-blue work well due to the very even luminance distribution across the entire viewing angle. Even better is TRANSMISSION rear projection screen, with its combination of high luminance and even luminance distribution across the entire viewing angle.

 

Perforation and Microperforation:

Perforation or microperforation is typically used to help alleviate acoustic pressure through a projection screen, particularly middle to high range frequencies. Standard perforation is 57,000 holes/m² (37 holes/in²) with an open area of approximately 7%; microperforation is 300,000 holes/m² (195 holes/in²) with an open area of approximately 6%. For a relatively close viewing distance, a microperforated screen is recommended. For farther viewing distances, a standard perforated screen such as white is recommended. Under proper viewing distances and conditions, perforation is not visible.

Moire Effect:

A moire effect is a secondary visual pattern, usually unwanted, created by a number of factors specific to the effect. It’s usually categorized as an optical waviness to the image and can be distracting to the viewer. In the case of a moire effect from a projected image onto a scrim, this is typically caused by a very high resolution projector with many pixels that has been superimposed onto an open weave pattern thus resulting in a wavy or vibrating image to the human eye. To alleviate this, large scale projection tests using the desired scrim fabrics should be done to see if the moire effect will happen.

 

Frame Projection:

Projection of data or images with a projector that illuminates the entire screen surface or “full frame.”

 

Cleaning:

It is recommended to clean PVC projection screens with a special solvent-free cleaner and microfiber cloth to ensure long life and durability.

 

Repair Kit:

A PVC repair kit is available to repair minor damage such as rips or tears. The repair kit consists of highstrength PVC adhesive and a 0.25 m² (10″ x 10″) piece of screen. Please indicate specific screen when ordering. Please note that repairs will always be visible, especially on rear projection screens.


Source: Garriets

 

 

 

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