Reflections on Mirrors

Some Reflection Terminology

There are two basic classes of reflections. "Diffusive" Reflection and "Specular" Reflection .

Diffusive Reflection

Most surfaces are not of a glossy polished finish that would reflect light in the manner we know as a mirrored reflection. Such surfaces still reflect incident light but they do this in unpredictable and multiple directions .Such a surface also absorbs rays of light that are the same colour as the surface. Most light directed to our eyes travels in this manner.

Specular reflection

This is the other form of light reflection and is what we normally mean when we discuss mirrors. This effect occurs on polished or glossy surfaces as well as the settled interface between water and air. In Specular reflection situations you can rely on the angle of incidence being the same as the angle of reflection from the surface.

A beautiful specular reflection found in nature - enjoy! Imaging the wind blowing ripples and you are envisioning the change to a diffusive reflection.

Refraction

At the surface of an optically clear or transparent substrate incident light will obey the normal rules of reflection. This reflection will occur at the interface between the surface and the air but some of this light will also be transmitted through the surface and out the other side. This light will obey the rules of "refraction". Here incident light is split into light that is reflected and light that is refracted and transmitted through the substrate . Sometimes this is called beam splitting.

Reflection and refraction in an optically clear block

Front Silvered Mirror or First Surfaced Mirror

These two phrases describe the same item and are interchangeable.

Bathroom style mirror set up

An ordinary household mirror e.g. a bathroom mirror has the reflective silver coating applied to the rear of the glass . This allows the delicate silvered surface to be protected from abrasive action which might happen during cleaning. The fact that the mirror is viewed square on means that the flaws in such a set up are not really exposed to the user. The reason is that there are no obvious unwanted secondary reflections exposed by the viewing angles . See the diagram: -

If you compare this bathroom mirror with a front silvered or first surface mirror set up in the same orientation you will observe a greater clarity in the image because the imperfections inherent in the glass have been removed from the optical path .

In the photo below you can see the sheer joy that is apparent when someone sees a perfect front silvered mirror image for the first time. The diagram that follows shows how the glass has been removed from the optical path exposing the pure but very delicate reflecting surface.

First Surfaced Mirrors really come in to their own when they are set up in an optical system which relies on angled reflections. Comparing the following diagrams reveals why the first surfaced mirrors are better at producing non ghosted and optically much clearer images . The ghosting only blurs the reflected image as other similar images are relayed over the top of the primary image.

Since most holographic illusions use mirrors in one form or another and that these mirror are almost exclusively held at an angle, then techniques for minimizing these aberrations are essential when produce a high quality installation. This is why we source and sell the very best First Surfaced Mirrors. They can be Glass Acrylic or Foil based First Surfaced Mirrors but each needs to be selected with knowledge of the components source.

If you are looking for these First Surface Mirror products for your Holographic or just a simple projection solutions please do not hesitate to call our staff on the number below. or if you prefer you can email hello@interestingav.com.

"Peppers Ghost" a special case of reflection

On 5th February 1863 Professor John Henry Pepper and co-inventor Henry Dircks entered the offices of Carpmaels, a London Patent attorney and together they filed patent No326 Apparatus For Exhibiting Dramatic And Other Performances. The patent described the set up of an illusion which had been built in private the month before in the Royal Polytechnic Institute, 309 Regent St London. This was the culmination of the work Henry Dircks had started a few years earlier and which was only made commercially viable by Professor Pepper's modifications. Six month later the patent was sealed and granted to the two engineers. During these early stages Pepper was assigned all the profits in exchange for a fee paid to Dircks. Although Professor Pepper states in his biographical report that he never claimed that the illusion should be named after him alone nor that Dircks should be forgotten, the inevitable consequence of Pepper's purchase of the rights and of his commercialisation of the illusion naturally led to his name, alone, being forever associated with this effect. You can read Peppers account in True History of the Ghost and All About Metempsychosis by John Henry Pepper - Cambridge University Press.

Not withstanding the fact that there are numerous documented reports of the history of the illusion, until recently there has apparently never been a precise technical definition of what a Peppers Ghost actually is. You can find much written about the colourful characters whose lives have been intertwined with the illusion and a few diagrams but not a concise explanation. Of course you can read the patent description Pepper and Dircks filed but even that does not properly account for the way the phrase is now commonly used. We have grappled with how to best explain the scientific meaning and offer the following explanation.

We can say for certain that A “Peppers Ghost”… is a type of reflection. And that not all reflections qualify to be called Peppers Ghosts… but all Peppers Ghosts do obey the scientific rules of reflection. As such:-

The angle of incidence of a ray of light always equals the angle of reflection. see specular reflection above.
The distance that the real image or object is in front of the reflecting plane = The distance that the virtual image appears to be behind the reflecting plane
But that does not seem to be comprehensive.

The following definition was first presented in May 2016 by our staff at Museum and Heritage show London 2016. We hope you like the definition and welcome improvements or even records of other earlier attempts at a scientific definition that encapsulates the way those skilled in the use the phrase... Do let us know if you can find better descriptions or similar that predates the following (...but not the historical accounts of John Henry Pepper - as they are easy to find and do not properly qualify as a scientific definition).

Definition of a Peppers Ghost Reflection

Happenstance

Peppers ghost reflections are a subset of all reflections. They are produced when there is:-

1. A light source e.g. an electronic display or a lit object which is defined as the “source image or object”.

2. This “source image or object” being hidden from view of the casual observer.

3. A reflecting material with a degree of transparency

4. a reflected or “virtual image”.

5. And importantly a background volume or space that the virtual image appears to sit within / interact with.

6. And this background volume or space being selectively lit so as to be obvious even without a virtual image.

E.g. on its perimeter to convince the viewer that there is an interaction between the virtual image and the real world of the background volume.
Sometimes the background volume is populated with real objects or real people which are lit at least some of the time.

7. There is some intent to use the virtual image created to either; inform, entertain or trick the viewer. Consider the difference between Happenstance and Intention as portrayed in these accompanying Photos.

Intentional

Beautiful Installation in Lincoln Museum - BRC

The following diagrams might also help here: -

The diagram to here shows the beam splitting properties of a "Peppers Ghost" transparent and reflecting plane.

By this means a person viewing from a position to the right of the diagram can see light from two different locations being mixed - note that the semi reflecting surface marked in the diagram would be; to all intents and purposes; invisible to the viewer.

The following diagram shows how the source image is hidden from the view of the observer. It also places the virtual image plane in its appropriate orientation to the source image and shows how it is possible to orientate a virtual image in a vertical manner. You can see the photo of the Darwin Museum set up where an actor playing Darwin appears to the audience in the museum as if he is interacting with the artefacts layout on the Beagles table.

One possible method for establishing a Peppers ghost floating virtual image

The Darwin House Museum in Kent UK , houses a scene based in Darwin's cabin in the Beagle. You can see an actor captured and reproduced within this set volume. He interacts with the scene in a fascinating manner really bringing the set to life.

Optical aberrations that can appear in Holographic Illusions and What can be done to resolve these issues: -

The strange case observed in reflections of LED wall panels.

Sometime ago we created a nice test with a very bright LED screen - a high quality Barco "black face" LED wall 5000nit P5 i.e. 5mm pitch between the LED pixels. the test was videoed and you can see the result in the following video. What you will struggle to see is the artefact that was much more obvious in real life. The zoomed in images of a frame from the video shows an area of interference pattern that made this illusion less attractive than it was expected : -

This observation really got us thinking about LED and its reflections. We found more evidence of this interference in reflections of perfectly good LED wall images in different reflecting surfaces. The next three Photos were taken in one of the Expo Pavillions in the Milan Expo 2015. It is a little hard to immediately understand what was happening. You have to imagine a room with several thick pillars covered in LED wall panels and the rest of the room i.e. wall floor and ceiling are covered in mirrored surfaces.

Green boxed areas helpfully define the area of LED wall i.e. and shows how good the image source is. Red boxes highlight the areas of reflected image that were to the eye of the viewer full of interference aberration.

You might at this point be thinking... so what!- you have shown that LED reflections are sometimes full of interference patterns. The reason we are concerned about this effect is that LED in a source of very bright image and is in higher light levels therefore a candidate for incorporation in Peppers Ghost illusions.

We continued our obsession for reviewing the performance of LED walls as reflections. One of our staff took this photo in New York and noted that the reflecting surface was glass. This is useful because we have seen the effect in our Peppers ghost foil and in acrylic and as shown here in glass.

Something causes this in some reflecting surfaces with some LED screens. It might be the fact that there are several point sources of light that are being reflected probably including internally reflected and this is forming a diffraction grating interference pattern.

This means to create a great quality Peppers ghost with a n LED screen you need to test components in advance or there needs to be a fix.

After several tests at the offices of LEDMan GmbH we developed a treatment for mis-performing LED based Peppers ghost systems. This treatment effectively eliminates the aberrations and is reasonably price.

Please ask for more information if you need pricing etc - hello@interestingav.com or call the number below.

The Curious Case of Interference in the Reflected Image of an LCD Based Projector set up in Rear Projection in a Peppers Ghost Illusion.

Ok we fully understand we are getting into the micro-niche areas of an already niche topic with this one. The take away for most people reading this section is as follows:-

DLP based projected images can be projected in front or rear projection mode and there will be no issue relating to the selection of this technology on the appearance of interference patterns in the reflected image so DLP technology is fully compatible with Peppers Ghost illusions.
- This is true for single and triple chip DLP projectors
LCD based projected images can in certain set ups cause unsightly interference patterns that ruin the illusion.
- LCD projected images that are set up in front projection mode can make very nice Peppers Ghost illusions
- LCD Projected images that are set up in rear projection mode will to some extent be compromised by interference patterns - we strongly advise not to use LCD Projection for this configuration.
  - This is true for standard and 3 -LCD technologies

We have no fix for this error and hence avoiding inclusion of such a projector in a rear projected Peppers Ghost design is the best option.

For the more curious amongst us it is a mystery that this should happen and a search for the reasons might lead to a removal of this technical block. - Of course we should not expect this to be a huge commercial opportunity and as such it is obviously hard to ask for resource to focus on a solution - there are just too many alternatives available for this to demand a budget. So this corner of the web is simply a nerdy hobbyist challenge. and if that does not describe you you probably wont want to read any further on this page. What we do know about this interference pattern

anyone we have discussed this with who is in a technical capacity in a projector manufacturer has struggled to explain the reasons for this effect.
the solution to the LED problem described above does not work when applied to this issue.
- but it is a similar looking effect.
The effect might be described as similar to an oil on water refraction or to a moire effect that is seen with similar sized holes in two meshes.

First surface mirror to lens separation - how to avoid unwanted optical aberration - Pratt's Golden Metre Rule.

We can sell our client's very large front surfaced mirrors eg 3.2m x 1.9m for use in large projection systems when the light-path needs to be folded. Over the years we have done a lot of work to establish how best to source and set up this type of component. I guess it is unsurprising that we have learnt that optical components need to be selected for them to work together in an optical manner. We have learnt the hard way that it is possible to use the same projector lens and one of our mirrors and for there to be different results depending on the distance that separates the components.

It is a strange effect but you can see it manifest as faint vertical lines in the first projected image especially noticeable in the white areas. the second image shows this effect to have gone. The only difference here was that we moved the projector to be much closer to the mirrored surface in the second image compared to the first.

For some people the effect is not sufficiently problematic to cause them concern but for any high end projection system (which is the usual client profile we have) this is not an acceptable fault.

We call the solution to this problem the "Pratt's Golden 1m Rule" -named after Steve Pratt who worked on the best work around to the issue.

This rule basically advises that without any other testing (which is still best) you can, pretty reliably, plan that your system will be optically acceptable; if the separation between the projector lens and the first surface mirror is less than 1m.

This rule works because it usually accounts for the root cause of this issue. This root cause being related to the lenses "depth of field of focus". This figure is baked into the design of a lens but is never stated in product specifications. This value relates in the way focus plane works. In past times projection screens were flat and perpendicular to the orientation of an associated projector therefore focus just needed to work on that one plane - provided there was a position on the focus dial that allowed the flat plane to be in focus, then all was good. Fast forward to the first decade of this century and suddenly people were looking at all sorts of ways to use projectors e.g. off axis, or with curved screens or with extreme keystone etc. You only have to think about a projection mapping project e.g. on the front of a building and you realise the advantage of opening up the depth of field of focus from a not very deep plane to a more forgiving deeper depth of focus .

With no obvious downside to opening up this depth of field of focus, the natural choice is push the limits to the max. In doing so some manufacturers have optimised their projectors in line with the larger market opportunities eg simulation on curved screens or projection mapping but unwittingly closed down the options for engineers making solutions in the very much smaller market for rear projection solutions. In these rear projection solutions it is common to want to place a mirror several m away from the lens. In doing so the mirror surface is positioned in some systems within the depth of filed of focus of a lens. At this point an otherwise perfect mirror surface is exposed to have errors and aberrations that are not obvious to an inspector not reviewing the mirror surface under projected light.

It is a perfectly obvious and sensible commercial choice to make a product work for the larger markets but for those of us in this niche market this trend in optics has closed down options for selecting projector manufacturers or for using these companies products in some setups.

The workaround is therefore first test your projector lens and mirror set up and in the absence of testing fall back on the "Pratts Golden 1m Rule".

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