Flare Vs Stray Light: In the academic and scientific literature, "Stray Light" is more generally meant to describe the cause of a phenomenon that affects the performances of imaging systems whereas "Flare" or "Veiling Glare" is meant to be the ill effects of this phenomenon on the image. In other words: stray light is the illness while flare and veiling glare are the symptom. Nevertheless "flare" (a.k.a "veiling glare") and "stray light" are considered to be synonyms in many popular magazines, reviews or tutorials on the Web, articles and Webinars.
So again, Flare or Veiling Glare means the effect and Stray Light should mean the cause. This semantic split between these two synonyms matches the difference between the concerns of their respective user's communities i.e. the imaging systems Designers (and manufacturers) on one sideVs the Users of these systems (e.g the photographers) on the other side. Of course some Professional groups actually interface with both sides (or alternatively they are concerned by both point of views) and consequently, they may use a mixed vocabulary incorporating more or less both notions in their literature (e.g. ISO Standards).
Flare Vs Veiling Glare: Historically Flare used to be more often selected amongst other synonyms, including veiling glare, until the late 1960's. After this era, the (now obsolete) ANSI Standard PH3.615.1980: "Method of testing veiling glare of still-picture camera objectives"noticeably preferred veiling glare to flare for its introduction in 1980. Since the word Flare was almost universal, its useage still persists in casual photographic literature. BTW this word is used "as is" in French, for instance. Nevertheless it is now more often being used in usual photography to give a name to multi-colored ghost images as well as to the star-like long streaks of light that appear sometimes on images shot against very bright lights. In this context, the expression veiling glare is consequently often -but not always- restricted to represent the veiling effect (haze) that may temper the general contrast of a photograph. Note: The use of the two terms also sometimes encompasses also the effect of "Blooming" for example when e.g. referring to CCD Vs CMOS sensor IQ. This happens more frequently in literature from the Camcorder and Display industry though.
*) Michael A. Gauvin and Edward R. Freniere wrote in Reducing stray light in Opto-Mechanical Systems (circa 2000) Note: some paragraphs found on this historical paper are reproduced in my main Flare related article. The original has been plagiarized dozen of times in scientific articles without credit to the authors.
Almost every optical imaging system suffers from stray light or unwanted light. In an optical imaging system, stray light is caused by light from a bright source shining into the front of the system and reaching the image as unwanted light. Stray light also occurs in non-imaging systems. Stray light is commonly manifested in two ways: ghost images and scattered light.*) In the outstanding Webinar "Photon Engineering "Stray Light " (May 2011) the authors write that Stray Light is Unwanted energy: like weeds on a lawn.. That is an superb parabl but this is hardly a definition, though:)
During the 61 slides presentation, the speaker neither defines the words group Stray Light (while using it hundreds of times) nor does he define Veiling Glare while it wrote these words on two slides. That cannot really be a surprise because the audience of the webinar is composed mainly of knowledgeable Optical Designers. Note: "Flare"is never mentioned at all in the 61 slides of this presentation!
*) At lastt, in another Photon Engineering's paper, Stray Light Application Note, one can read what could eventually be a definition,
Stray light is simply unwanted noise (light) that is added by either opto-mechanical structure, out of field sources, non-perfect optical components or through thermal emission in an optical or illumination system.
*) SMIA 1.0 Part 5(Standard Mobile Imaging Architecture) : § 5.24 Camera Characterization Specification (2004)
(5.24 Veiling Glare 5.24.1 Description) Veiling glare is the reduction in contrast, or “misting”, in an optical system due to random scattering of light onto the image plane. Note: AFAIK SMIA effort was discontinued when the CPIQ Initiative started (read next paragraph).
*) CPIQ Initiative Phase 1 White Paper: § Fundamentals and review of considered test methods (Camera Phone Image Quality -Initiative, October 2007)
(p 73: Glossary of terms): Veiling Glare (Camera Flare): relatively uniform but unwanted irradiation in the image plane of an optical system, caused by the scattering and reflection of a proportion of the radiation that enters the system through its normal entrance aperture NOTE 1: The veiling flare radiation may be from inside or outside the field of view of the system. NOTE 2 : Light leaks in an optical system housing can cause additional unwanted irradiation of the image plane. This irradiation may resemble veiling flare. [See ISO 9358] Please note another synonymous group of words (Veiling Flare).
(p 33: 5.1.5 Objective camera Flare test example): Camera flare is the additive contribution to focal plane illuminance from light that is scattered in the image forming process, and internally reflected within the camera. It is more apparent when there are large, extended sources of high brightness within the imaged scene. The impact of flare is to reduce the overall contrast of an image. It occurs to varying degrees in all images, producing an increase in the apparent luminance of all objects within the scene. Perceptually, flare more significantly affects the dark areas of the image because it makes up a larger proportion of the exposure in these regions. Flare is also locally varying within the image, and can even form patterns; however, from a practical standpoint only the average flare is typically determined...
*) ISO 9358 (1994): Optics and optical instruments– Veiling glare of image forming systems – Definitions and methods of measurement
Veiling glare is the sum of individual stray light contributions to a single pixel from all other light from the scene, including light beyond the camera’s field of view. This standard defines the glare spread function (GSF), which is a measure of stray light per angle... The veiling glare index (VGI) is defined as the ratio of the irradiance at the centre of an image of a small perfectly black area superimposed on an extended field of uniform radiance, to the irradiance at the same point of the image plane when the black area is removed.
One should note however that unlike ANSI Standard PH3.615.1980, ISO 9358 (1994) Standard doesn't address the photographic instrument specifically.
*) DxOMarks: Glossary
/Flare: Optical artifact coming from reflection on lenses, usually visible with a bright light source (such as the sun). Can lead to “ghosts” of the light source or “veiling glare” (loss of contrast) that creates a foggy effect on the image.Canon
Canon had posted a quite extensive, excellent and informative e-Book on its EU website. One can read some definitions such as the following:
Light reflected from lens surfaces, theinside of the lens barrel and the inner walls of the camera’s mirror box can reach the film or image sensor and fog part orall of the image area, degrading imagesharpness. These harmful reflections are called flare. Although flare can be reduced to a large extent by coating the lens surfaces and using anti-reflection measures in the lens barrel and camera, flare cannot be completely eliminated for all subject conditions. It is therefore desirable to use an appropriate lens hood whenever possible. The term “flare” is also used when referring to the effects of blurring and halo caused by spherical and comatic aberration.
Michael Reichmann, the author of The Luminous Landscape, wrote in one of his excellent tutorials Understanding Lens Contrast And the Basics of MTF:
... (A Third Definition): ...
... The big question mark where local contrast is concerned is that almost all actual picture-taking situations allow flare and veiling glare (the latter an overall dulling or haze of the image similar to "flashing" an enlargement with a low dose of non-image-forming light, or fog) to contribute in varying amounts and varying ways. Despite lots of scientific research, there still seems to be not much way to quantify it exactly, or predict its contribution exactly with any given system ("system" meaning camera-lens/film/enlarger-lens/paper) in real-world situations. Flare is always present to at least some degree, but it is seldom present in exactly the same way in two different systems encountering two different situations.
... Have you ever noticed how many early 35mm photographers tried to avoid bright sunlight? You might be forgiven for thinking that the decade of the 1940s was entirely overcast (and not just by the world political situation). With experience as their teacher, many photographers in the '30s and '40s learned various clever ways of avoiding or minimizing high-flare situations. The amateur admonition to "never point the camera in the direction the sunlight is coming from" dates from this era. Such was life with "miniature" cameras before the days of multi coating.
The last paragraph summerizes very well why in practice ghost images were seldom a problem in earlier days of photography: like for the painters, the photographic studios were located with large windows on the northern part of buildings. No one was willing to waste films or plates by shooting "à contre-jour" (against the light). The last sentence implicitly assumes that some photographers use "non-miniature" cameras. That is the case for example of big and costly imaging systems produced and used for institutional purpose: They shall never be shooting against the sun or toward a powerful specular lighting (e.g. The Navy recon planes do not shoot right above the equatorial ocean surface at 12 o clock). The problem of flare for them is totally different from the panorama photographers: they fear from pure "lens veiling glare" and most of the Scientific literature and Standards were written by and for these high-end photo user's group... then the cheap low-end CMOS camera became available and especially the Mobile Phone users -and telecom providers- flocked in: first SMIA works and then CPIQ initiative were then started in a hurry... sadly no real study was done to help the casual DSLR user:(
That said, words like Flare, Veiling Glare, Camera Flare, Stray Light,Veiling Flare, Veiling Haze or even Ghosting Flare (!) may have of course, like I have said above, a different meaning for different users groups often because they respectively design, build, promote, sell or use possibly completely different imaging means for possibly very different purposes.The same words may thus mean different things and vice-versa different words may mean the same thing. Moreover, the practical meaning of these words has also evolved in time with... evolving technologies. For instance better baffling and more efficient flocking of surfaces against scattered light, multi-coating on optical glass surfaces, multiplication of glass elements for (zoom) lens construction, etc., have somewhat shifted the focus of concerns from "Please, give me a little more contrast" to "We don't want to see those ghost images, no more, damn it".
At last, to demonstrate the fundamental difference in the Point of View which exists between the two groups (Flare Vs Stray Light), I would like to quote Lambda Research Corporation that very recently promoted using TracePro® for Ghost and Stray Light Analysis: On the February 2012 Webinar handouts (page 35) one can read:
Four Methods to Reduce Stray Light
Moving the stray light by tilting a lens, moving the detector, or angling the offending stray light surface is the best way of getting stray light out of a system. You may need to add a beam dump to completely get rid of the problem.
Even when stray light sources do not illuminate the optics directly, they can still scatter from structure or baffles and then illuminate the optics. Using baffles is a great way to stop out of field sources from sending light directly to the detection device.
Usually occurs when a shiny object is illuminated by a stray light source. Coating the offending shiny object with black paint usually reduces this stray light quite substantially but not completely to 0. For instance, black anodized aluminum can be 35% reflective but there are better black paints available several are in the 3 percent range but there can be problems with out gassing and degradation over time with these coatings. Some portion of the light will always scatter into the field of view and become stray light even with the best of coatings. Set a tolerable specification
Especially important to get rid of ghost images -Ghost images are out)of)focus images of bright sources. Light must reflect an even number of times from lens surfaces. If the source is small each ghost looks like the aperture stop. If the ghost is focused on the image plane, the ghost looks like the source. To get rid of ghost images we coat the lenses with anti)reflective coatings to reduce ghosts
These recommendations clearly can hardly be implemented by the casual photographer, end user of the imaging system with the exception of the second one: the hood that comes in the box of the lens (or the built-in integrated sun shade if it existed) may be of some help. I particularily like the advice given by Canon to the users of the EF TS-E24 mm f/3.5 L MkII:
The EW-88B lens hood can keep unwanted light out of the lens, and also protect the lens from rain, snow, and dust.
The Instruction for using the new Canon 8 mm f/4 L USM fisheye zoom lens is also of interest as one can read on page 11:
Vignetting shall not occur while shooting in the following zoom ranges when using the (EW-77) lens hood. Please detach the lens hood when shooting outside these ranges.
What else could Canon have said to tell the photographer that when the bright light source is out of the field but yet getting through the entrance pupil of the lens, the hood mounted or not, (from experience, that seems to be the case for all the lenses that I own) there is only a piece of cardboard to possibly can save a shooting day? But then why does not Canon tell the same for all the lenses that he sells (including the 8-15 mm)?