Retroreflective Screen = Curved Screen - AVS Forum
Forum Jump: 
Closed Thread
 
Thread Tools
post #1 of 45 Old 01-22-2002, 08:11 PM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Neither hotspot for the same reasons despite having high gain.

The Mothership is now boarding.
MrWigggles is offline  
Sponsored Links
Advertisement
 
post #2 of 45 Old 01-22-2002, 08:35 PM
Advanced Member
 
KFung's Avatar
 
Join Date: Apr 2000
Location: Ottawa, Ontario, Canada
Posts: 879
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
<caveat> Only in applications where the projector is close to audience...

Curved screens are more flexible in their effects, but quite a bit harder to pull off. Off-axis viewing will usually be better with a curved screen.

Regards,

Kam Fung
KFung is offline  
post #3 of 45 Old 01-23-2002, 09:12 AM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Explain.

The Mothership is now boarding.
MrWigggles is offline  
post #4 of 45 Old 01-23-2002, 11:05 AM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Actually, if you are able to place the projector (and viewer) vertically centered, you will have a setup that for all practical purposes (brightness, brightness variations and color shift) equals that of an optimized torus screen - even for horizontally off center placed viewers. However, as soon as you ceiling mount the projector, the brightness will be a lot lower for the retro reflective flat screen, some 20-30% lower (8' ceiling, depending on horizontal position) for a 2.8 gain material. With a viewing position slightly lower than screen center (10-15"), the brightness will be lower still, some 30-50% lower than the torus. Brightness variations and color shift will still be approximately equal regardless of horizontal position, though.

In the above the gain curves are considered to be equal for both types of reflectivity. PJ CRT NEC135LC, a long throw digital PJ will be a better match for the retro reflective screen if ceiling mounted.
Iceman is offline  
post #5 of 45 Old 01-23-2002, 11:22 AM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Iceman,

I was working in the horizontal dimension to begin with. And your points about ceiling mount and long throw are correct.

However, with many digital projectors currently out with lens shift and long throws, a good compromise can be reached as far as placing the projector at just the right height and using a retroreflective screen.

It is new way of thinking for those who have digital projectors and don't want the geometry distortion created with a curved screen.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #6 of 45 Old 01-23-2002, 11:50 AM
AVS Special Member
 
jamoka's Avatar
 
Join Date: Oct 2001
Location: Tullahara
Posts: 1,749
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Have any of you "experts" measured, or even seen, the hotspotting on a high gain retroreflective screen?

(Clue: it's not zero.)

Joe
jamoka is offline  
post #7 of 45 Old 01-23-2002, 01:32 PM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
MrWigggles,

I believe a short throw PJ will work better than a long throw if vertically centered. This is self-evident for a horizontally centered viewer but is probably also the case for off-center viewing positions. I will run further simulations tomorrow so I can present some numbers.


jamoka,

Your humble manners continue to amaze me. If the screen material truly behaves as advertised, i.e. perfectly retro-reflective, a vertically and horizontally centered setup will produce no noticeable hot-spot. I am sure this may not always be the case in practice for all retro-reflective materials.
Iceman is offline  
post #8 of 45 Old 01-23-2002, 01:42 PM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally posted by jamoka
Have any of you "experts" measured, or even seen, the hotspotting on a high gain retroreflective screen?

(Clue: it's not zero.)

Joe
Your are correct.

it is 0.0000045563

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #9 of 45 Old 01-23-2002, 02:05 PM
AVS Special Member
 
jamoka's Avatar
 
Join Date: Oct 2001
Location: Tullahara
Posts: 1,749
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
"Your humble manners continue to amaze me. If the screen material truly behaves as advertised, i.e. perfectly retro-reflective, a vertically and horizontally centered setup will produce no noticeable hot-spot"

Exactly. IF yada yada yada is true.... THEN blah blah blah. But we all know at the outset yada yada yada is NOT true. So what's the point?

Your humble servant,
Joe
jamoka is offline  
post #10 of 45 Old 01-23-2002, 02:34 PM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Joe,

Would pictures or light measurements satisfy you?

Geometry does come into play. Retroreflective screens converge the light path back to the viewers while normal reflective surfaces have reflected light that diverges away from the viewer unless the screen it curved.

I could explain it further but I don't think it would satisfy you so I will stop here.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #11 of 45 Old 01-23-2002, 03:06 PM
AVS Special Member
 
jamoka's Avatar
 
Join Date: Oct 2001
Location: Tullahara
Posts: 1,749
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Thank you.
jamoka is offline  
post #12 of 45 Old 01-24-2002, 04:21 AM
Advanced Member
 
Robin's Avatar
 
Join Date: Jun 2000
Location: Michigan
Posts: 578
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Mr. Wigggles,

Is there a retroreflective screen being manufactured? If so details please. I have long been interested in this concept.

Thanks!
Robin
Robin is offline  
post #13 of 45 Old 01-24-2002, 04:37 AM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
MrWigggles,

Using a throw distance significantly longer than the viewing distance will naturally make for a less than optimized horizontally centered viewing position. For horizontally off-center positions a shorter throw distance will provide a much better brightness uniformity, up to some 30% less variation (2.8 gain, viewing distance 1.5 x screen width, PJ distance 1.5 x screen width vs. 2.5 x screen width vertically centered). The longer throw distance will have a much higher peak brightness for off-center positions, but the brightness variations will be way too high IMO if you sit level with the screen edge at 1.5-2 screen widths from the screen.



Quote:
Exactly. IF yada yada yada is true.... THEN blah blah blah. But we all know at the outset yada yada yada is NOT true. So what's the point?
jamoka,

I have seen retro-reflective screens working as specified but as I know you will not take my word for it, why bother? Since you seem to have seen a retro-reflective screen behave poorly under some circumstances, you will never listen to reason anyway.
Iceman is offline  
post #14 of 45 Old 01-24-2002, 10:37 AM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
ICEMAN,

The viewers position makes a difference of coarse. If the viewer is 1.5 X away then that is the optimal viewing location as well.

However, if the viewer was to move about the room staying a constant 2X away from the screen, both projection locations would be very good but the 2.5X location would have slightly more left to right uniformity than the 1.5X location.

Robin,

All glass beaded screens are retroreflective. The normal problem with these screens is that they tend to sparkle do to the size of the beads. The Da-Lite Hi-Power which uses micro fine beads produced by 3M doesn't sparkle so for pratical purposes that is what I am refering to in this discussion.

For ceiling mount CRT users who have great control of their geometry, a curved screen with a normal gain surface is the best way to go.

For table top mount Digital projector users, the retroreflective screen is the best method.

For ceiling mount digital users, I think it depends mainly on how high the person is projecting from as to what his best choice would be. The closer to horizontal the more retroreflective is the best choice.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #15 of 45 Old 01-24-2002, 02:23 PM
Advanced Member
 
Gunnar's Avatar
 
Join Date: Oct 1999
Location: Lillehammer, Norway
Posts: 933
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Will a curved screen with retro reflective material behave very differently from a curved screen with reflective material? My HT room will (eventually..) be located in the basement, and due to the low ceiling I`m considering table top mounting the projector. Since I want to experiment with a curved screen I thought that retroreflective material would be a natural choice?

Gunnar
Gunnar is offline  
post #16 of 45 Old 01-24-2002, 02:34 PM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Gunnar,

There is essentially no good reason to use a retroreflective surface on a curved screen, unless curving your screen somehow helps your geometry which is highly unlikely if you have a digital projector.

If you table top and have fairly low lumen projector (i.e. 800 or less "DLP lumens"), then a retroreflective surface is a great option.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #17 of 45 Old 01-24-2002, 02:42 PM
Advanced Member
 
Gunnar's Avatar
 
Join Date: Oct 1999
Location: Lillehammer, Norway
Posts: 933
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Thanks!
I will probably use a BG-1200 CRT with about 1000 lumens. I`m considering a curved screen, because I like the idea of having a large screen with a uniformly bright picture, and minimal color shift. Maybe I should reconsider the table top idea?

Gunnar
Gunnar is offline  
post #18 of 45 Old 01-24-2002, 10:15 PM
Advanced Member
 
KFung's Avatar
 
Join Date: Apr 2000
Location: Ottawa, Ontario, Canada
Posts: 879
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Mr. Wigggles,

Sorry it took so long to reply, good thing Iceman is here! A curved screen is more flexible in its effect because you can use the curvature to control the direction of the reflected light, much like you would by varying the throw distance. You also have a degree of control over the gain curve because you can choose different gains of reflective fabrics, the selection for retro-reflective screens is rather less by the nature of the material. This allows a good deal more flexibility in placement of the audience and the projector (although longer throw is almost always better with a curved screen). With a retro-reflective screen you are forced into a more restrictive placement for your projector to get good results or good light efficience, it's a good option if you have this kind of control. The only problem is the projector sitting very close to your line of sight.

By virtue of their higher gain you will naturally get more non-uniformity (outside of the optimum viewing position) than with a curved reflective screen of lesser gain. Unless you need the high amount of gain that a glass beaded screen provides it is generally better to go with a lower gain (1.8 or so) curved screen, as long as it doesn't cross reflect too badly.

Regarding off-axis viewing. I agree with both you and Iceman (no big surprise :D ). However I believe Iceman's simulation was for a horizontal row of viewers, in this case a shorter throw would be better because the it directs the light from the far sides of the screen "across" the rows. Once you start moving back, as in your example, the longer throw should provide a more uniform image, but in a narrower viewing cone.

That said the retro-reflective screen is great if you have a narrow cluster of viewers (whether in the left-right or front-back direction) and it's a lot easier to deal with than a DIY curved screen.

Regards,

Kam Fung
KFung is offline  
post #19 of 45 Old 01-24-2002, 10:33 PM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Kfung,

I was trying to keep things somewhat hypothetical in nature.

A 2.8 curved versus a 2.8 retroreflective should look quite similar.

I do wish that a 1.8 retro was made; but it is interesting that you have to go pretty far off axis for my 2.8 to go less than 1.0. Unfortuneately, once you see 2.8, 1.0 stinks :)

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #20 of 45 Old 01-25-2002, 05:20 AM
AVS Special Member
 
wireburn's Avatar
 
Join Date: Feb 2000
Location: Muskegon, Michigan
Posts: 1,355
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Mr. Wiggles,

Here's an interesting point of information for you. The Dalite Hi-Power never goes below 1.0 gain, even at 90 deg. off-axis. The simple reason is because the base material is matte white (perfect diffuser). A more in-depth discussion is available at the Dalite website.

-Mike

I didn't do it
Nobody saw me do it,
You can't prove a thing

An Haiku by Bart Simpson
wireburn is offline  
post #21 of 45 Old 01-25-2002, 07:36 AM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
However, if the viewer was to move about the room staying a constant 2X away from the screen, both projection locations would be very good but the 2.5X location would have slightly more left to right uniformity than the 1.5X location.
MrWigggles,

Not quite true as it is really the other way around. The 1.5X location has a brightness ratio of 1.33 (2.8 gain) for a viewing position level with the screen edge, while the 2.5X has a BR of 1.53. A small but significant difference that is easily seen in the pictures below. Another very interesting result is that the bright side of the screen shifts from left to right when the PJ-screen distance increases! Not very strange when you think about it, but quite exciting anyway.

The viewing position is placed level with the screen right edge and the distances are the projector-screen distances. The viewing distance is constant at 2X screen width (seating row distance).




Full white retro-reflective 2.8 gain, PJ distance 1.5X screen width, off-center 0.5X screen width

http://w1.847.telia.com/~u84704441/S...W28RF1-PD3.jpg


Full white retro-reflective 2.8 gain, PJ distance 2.5X screen width, off-center 0.5X screen width

http://w1.847.telia.com/~u84704441/S...W28RF1-PD5.jpg



Quote:
The Dalite Hi-Power never goes below 1.0 gain, even at 90 deg. off-axis. The simple reason is because the base material is matte white (perfect diffuser).
wireburn,

??? This would make the screen reflect quite a lot more than 100% of the incident light. If I am not misinformed, the Hi-Power goes below unity gain somewhere around 30 degrees off-axis.
Iceman is offline  
post #22 of 45 Old 01-25-2002, 07:57 AM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Unless you need the high amount of gain that a glass beaded screen provides it is generally better to go with a lower gain (1.8 or so) curved screen, as long as it doesn't cross reflect too badly.
Kam,

I made a few off-hand calculations regarding 2.0 gain vs 3.2 gain materials and came to the conclusion that you probably should not go below 2.0 gain for a deep, optimized, curvature torus screen. The worst case cross reflection ratio (CRR) would be around 25 for the 2.0 gain material while it would be >700 for the 3.2 gain material. Naturally, any practical problem will only occur for high light situations in the screen corners, something that perhaps will minimize subjective impact and allow for quite poor CRR's.
Iceman is offline  
post #23 of 45 Old 01-25-2002, 09:16 AM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Wireburn,

A screen can not generate light. If it has >1.0 screen gain on-axis, it has to have some <1.0 screen gain off-axis.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #24 of 45 Old 01-25-2002, 09:24 AM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Iceman,

I wasn't clear. The 2X was the viewing distance. The "1.5X location" and "2.5X location" were the projector locations.

I was moving the projector forward and back in my example while keeping the viewer 2X away.

Nice pics.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
post #25 of 45 Old 01-25-2002, 10:17 AM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Ooooooops! That IS the projector distance that varies. I have edited the post for getting rid of the errors. Sorry for the sloppy description of what is actually shown in the pics.

I guess having your one-year-old at your feet while working can lead to brain glitches :D.
Iceman is offline  
post #26 of 45 Old 01-25-2002, 12:27 PM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Also, note that this is really nit-picking. Even the higher BR above of 1.53 is less than for a horizontally centered viewer with a normal ceiling mount setup and a 1.3 gain Stewart Studiotek screen (BR = 1.69, although the lower brightness is closer to the corners in this case, making it somewhat less of a problem in practice). As long as the BR stays below 2 or so, you will usually not perceive any objectionable brightness variations across the screen under normal viewing circumstances. The full white field makes differences more dramatic than in practice, sort of like listening to test signals on your sound system.

This thread contains lots more about these issues:

http://www.avsforum.com/avs-vb/showt...hreadid=104630
Iceman is offline  
post #27 of 45 Old 01-25-2002, 02:41 PM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Wow, now that we are on the same page.

All I can say is that practice is differing somewhat from theory. :)

ICEMAN, Your observation about the peak intensity switching from left to right is correct which is clearly shown in the pictures. It's your BR's for the 1.5X and 2.5X that seam to be reversed (I know they aren't but they seem that way)

Looking at the problem theoretically:

Theory-wise the center line for peak illumination would be a line directly back to the projector. This I will call the Return Path. The difference angle between the return path and the actual viewing angle I will call the Difference Angle

Given the 2X off-center seating position described above. For the projector mounted at 1.5X, the viewer has Difference Angle of 8.1 degrees from the return path of the far left side of the image and a Difference Angle of 18.5 degrees from the return path of the far right side of the image. Thus the left side of the image will be brighter than the right side of the image as shown in the picture above.

Now, with the projector mounted at 2.5X away, the Difference Angle to the left side becomes 15.3 and the difference angle from the right side becomes 11.3 degrees. Thus the right side will be brighter as the pictures above indicate.

There is a 10.4 degree difference in the two sides with a 1.5X throw and a 4 degree difference in the two sides with a 2.5X. Therefore the uniformity should be better on the 2.5X distance, THEORETHICALLY.

However, the screen isn't behaving to theory so I would suggest that it is not completely retroreflective. i.e. some of the light reflects off in a normal manner.

Very interesting data. Thanks ICEMAN for the empirical results.

-Mr. Wigggles

PS. I think we both agree that having the viewing distance as the same as the projection distance will give any even better image for our off axis viewer.

The Mothership is now boarding.
MrWigggles is offline  
post #28 of 45 Old 01-26-2002, 03:21 AM
Senior Member
 
Iceman's Avatar
 
Join Date: Oct 2000
Location: Stockholm, Sweden
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
MrWigggles,

I think I have been unclear when it comes to the basis of my results. All of my results are from ray-tracing simulations. I believe the main difference is that my algorithm calculates the illumination at the viewing position from each screen element (some 20 000 of them in this case), meaning that I also account for the varying distances. In this case, the viewer-edge distances offset the angular influence and changes the optimal PJ distance. The algorithm is also calculating 3D so even the angles differ slightly.

A more complete description of the calculations can be found in the thread I refer to above.

Once again, sorry for the ambiguity.
Iceman is offline  
post #29 of 45 Old 01-26-2002, 10:08 PM
Advanced Member
 
snaggs's Avatar
 
Join Date: Feb 2001
Location: Perth, Australia
Posts: 594
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Are you saying, with a ceiling mounted projector, I can get good results with a retro-reflective screen ?

What impact will putting a tilt on the screen have ? (ie. leaning it forward) ?
snaggs is offline  
post #30 of 45 Old 01-27-2002, 01:38 AM - Thread Starter
AVS Special Member
 
MrWigggles's Avatar
 
Join Date: Aug 2000
Location: Houston, TX, USA
Posts: 6,240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Iceman,

If your result is a simulation, I would recheck your equation. If there is something wrong with my angles then let me know.

I am not sure how you are modeling the hi-power I don't have any exact info on it. But the fact remains that there is a greater difference with the angles when the projector is is closer at 1.5 than 2.5.

I don't have a CAD program with me at home or I would draw it out. I can do it in MATLAB if you want as well.

Thanks, your efforts on the normal reflective surfaces was right on.

-Mr. Wigggles

The Mothership is now boarding.
MrWigggles is offline  
Closed Thread Screens

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off