PPD Calculator
Notes:
The PPD (Pixels Per Degree) is the number of pixels per degree of the viewing angle. It can represent how noticeable (or unnoticeable) the pixel structure is. For example, if the PPD is 33.51 on a 1080p DLP display viewed from 1.0W, and the PPD is 33.51 on a 480p DLP display viewed from 2.25W, then the pixel structure is equally noticeable. The pixel structure becomes less noticeable if the PPD is increased, which can happen as a result of increasing pixel count, decreasing screen size, and/or increasing viewing distance. Conversely, the pixel structure becomes more noticeable if the PPD is decreased, which can happen as a result of decreasing pixel count, increasing screen size, and/or decreasing viewing distance. When the pixel structure is noticeable, it is commonly called the "screen door effect" (SDE). When DLP displays are used, for example, the pixel structure is usually noticeable when the PPD is under 30, and it is usually not noticeable when the PPD is over 35. A PPD between 30 and 35 is borderline in that the pixel structure may or may not be noticeable. When other types of displays are used, the pixel structure may be more noticeable if they have larger gaps between pixels, or less noticeable if they have smaller gaps between pixels.
The CVD (Critical Viewing Distance) is the viewing distance where the PPD optimally matches the resolving capability of the human eye. With 20/20 vision, the eye can resolve a critical detail that subtends as little as one arc-minute, i.e., 1/60th of a degree. Since it takes at least one pixel to represent a critical detail, the PPD is 60 at the CVD. When the actual viewing distance is less than the CVD, the eye still resolves just as many details (and perhaps resolves them more easily), but the eye might perceive less sharpness. When the actual viewing distance is greater than the CVD, the eye still perceives just as much sharpness (or perhaps even more sharpness), but the eye might resolve fewer details. [1]
The PVD (Preferred Viewing Distance) is the viewing distance that is chosen by viewers according to their personal preferences. Viewers typically prefer smaller viewing angles for smaller screens and larger viewing angles for larger screens. The PVD is estimated as an average. In other words, about half of the viewers prefer viewing distances that are less than it, and the other half prefer viewing distances that are greater than it. [2, 3]
NHK is a broadcaster in Japan that developed HDTV, which has an aspect ratio of 16:9. They recommend that the HVA (Horizontal Viewing Angle) be at least 30 degrees in order to achieve the so-called "induction effect." This effect occurs when the human visual system responds to the image as if it were observing a real-world scene. When the HVA is at least 30 degrees, simple eye movements are fully induced. To improve the sensation of reality, the HVA may be increased beyond 30 degrees in order to induce head movements and reduce the viewer's awareness of the screen itself even further. In addition, the viewing distance may be increased beyond 10 feet in order to weaken the viewer's stereoscopic ability to judge the actual distance to the screen, since this ability counteracts the illusion of depth in the image. To complement the effect, the image should be free of artifacts and have a resolution of at least 22 cycles per degree in order to be perceived as sharp. [4, 5]
20th Century Fox is a film studio that developed an anamorphic format called CinemaScope. During research in 1953, they determined that the ideal HVA for CinemaScope is 45 degrees at the prime seat, which provides the optimal compromise between picture involvement and picture quality. Although anamorphic formats have used various aspect ratios over the years, as shown in this table, the modern version (now called Scope) has a standardized aspect ratio of approximately 2.39:1, nominally referred to as 2.40:1 (pronounced "two four oh") or 2.35:1 (pronounced "two three five"). The various viewing distances for Scope are shown in this diagram. [6, 7]
THX is a company that certifies commercial theaters that meet certain criteria. The THX Cinema Certification Criteria states: "The recommended audience viewing angle for the Cinemascope image (2.39:1) from the farthest seat in the auditorium is 36 degrees. The minimum acceptable angle is 26 degrees." It is important to note that the angle is from the farthest seat, and it is recommended to be equal to 36 degrees. In other words, if the angle from the farthest seat is greater than or less than 36 degrees, then it does not exactly meet the recommendation. However, as long as it is not less than 26 degrees, it meets the requirement for certification. The angles from closer seats are implicitly recommended to be greater than 36 degrees, as shown in this diagram. [8, 9, 10, 11, 12]
SMPTE is an organization that develops technical standards for the motion picture and television industries. The SMPTE 196M standard states:
"All observers in a review room shall be located within a standard observing area which shall be:
within the limits of a 15° angle on either side of a perpendicular to the center of the screen, in both the horizontal and vertical planes;
at a distance of 3 picture heights ± 1 picture height from the screen."
It is important to note that the angle is from the center of the screen (not from the seat) and limits how far off-center the seat can be. Another document is EG 18-1994 (Engineering Guideline 18: Design of Effective Cine Theaters, reissued in 1994), which recommends a minimum HVA of 30 degrees, but it has been withdrawn by SMPTE as of March 5, 2003. [13]
References:
[1] HDTV displays by Jukka Hamalaien
[2] Influence of Display Size and Picture Brightness on the Preferred Viewing Distance for HDTV Programs by M. Ardito
[3] Recommendation ITU-R BT.500-11. Methodology for the subjective assessment of the quality of television pictures Section 2.1.2 of Annex 1, page 4
[4] A Viable Transition To DTV For Everyone--Not HDTV For Everyone by Craig Birkmaier
[5] Psychophysical Analysis of the "Sensation of Reality" Induced by a Visual Wide-Field Display by T. Hatada, H. Sakata and H. Kusaka, SMPTE Journal, Volume 89, pages 560-569, August 1980
[6] Technical Guidelines for Dolby Stereo Theatres Section 6.1 Screen Size, pages 51-53 (PDF pages 67-69), and figures (PDF pages 73-74)
[7] Screen Size: The Impact on Picture & Sound by Ioan Allen
[8] THX Cinema Certification Criteria Viewing Angle
[9] THX Certified Cinema Design Screen Placement
[10] THX Theatre Alignment Program - Recommended Guidelines For Presentation Quality and Theatre Performance For Indoor Theatres Section III. 5. Screen image size and viewing angle distortion (PDF page 10)
[11] THX Architectural Suggestions Section II. D. 3. a) Horizontal Viewing Angle, page 6 (PDF page 11), and a figure (PDF page 15)
[12] How THX Works by Jeff Tyson
[13] SMPTE 196M Motion-Picture Film - Indoor Theater and Review Room Projection - Screen Luminance and Viewing Conditions Section 9 Review room viewing conditions, page 2 (PDF page 98)
Diagrams:
This web page was created by Erik Garci.