The overlay
An overlay is when you put one OHT on top of another one. Because OHTs are transparent, what is on each one is shown
together on the screen. This is useful when checking answers to questions. You write the questions on one OHT, leaving a
space underneath. Then put the second OHT on top and line them both up. In the spaces you left after or to the side of the
question, write the answer s, in a different colour if possible. In the class, first you display only the questions. Then when
you are ready, you put the answers OHT on top, being sure to line up the two OHTs. The students can then see each
question followed by its answer.
Te ch n o l ogy
This same technique can also be used for gap-fill exercises. Write a text or sentences with missing words on one OHT and
write the missing words on the second. In the class you display the first OHT and when ready for the answers you overlay
the second. If they are lined up correctly, as described above, the answers will sit clearly in the gaps.
2. The reveal
This technique involves covering part of the OHP with a plain sheet of paper. Because paper is not transparent, any text on
an OHT under the paper cannot be seen. Instead of using an overlay to show the answers to questions, you can have all
the information on one OHT. Line up the piece of paper so it is covering everything except the first question and answer.
When you are ready to go on, move the piece of paper down to reveal the next question. Using this technique means that
the students aren’t looking at all the questions at the same time. You can focus only on the question you want to.
The OHP is a very useful classroom aid but there are a number of practical considerations to think about when using one.
• Too much information!
Although the OHP is a very clear and convenient way of displaying information, try not to use it as the basis of a
classroom lecture. If everything is shown on your OHT the students might get bored because all the information is there
and they won’t need to listen to you. So, either use it with the blackout technique and just reveal each point as you’ve
talked about it, or put it up at the end of your lesson as a reminder and a summary.
• Placement and focussing
For the projected image to be clear and easy to read, the OHP has to positioned carefully and focussed manually. It’s best
to set this up, if possible, before the start of the class.
• Bulb
Many OHP’s have a spare bulb inside the body of the projector. It is not uncommon for OHP bulbs to ‘blow’. If this happens
in the class all is not lost if you know how to find and change the old bulb for the replacement one.
• Pens
There are many different coloured pens available for use on OHTs. They come in different thicknesses. Be careful though as
some pens are ‘permanent’ - this means that once you write on an OHT you cannot erase what you have written. If you
want to be able to clean and reuse OHTs be sure to use a temporary marker.
• Copying on to OHTs
It is possible to obtain OHTs which you can use in a photocopier. However, normal OHTs will cause great damage to a
photocopier. If you want to photocopy on to an OHT, make sure you have the right kind!
• Light
As the image from an OHP is projected light, it is often necessary to darken the room so that the information can be seen
clearly. When using an OHP be prepared to turn off some or all of the lights and close curtains and blinds.
An overlay is when you put one OHT on top of another one. Because OHTs are transparent, what is on each one is shown
together on the screen. This is useful when checking answers to questions. You write the questions on one OHT, leaving a
space underneath. Then put the second OHT on top and line them both up. In the spaces you left after or to the side of the
question, write the answer s, in a different colour if possible. In the class, first you display only the questions. Then when
you are ready, you put the answers OHT on top, being sure to line up the two OHTs. The students can then see each
question followed by its answer.
Te ch n o l ogy
This same technique can also be used for gap-fill exercises. Write a text or sentences with missing words on one OHT and
write the missing words on the second. In the class you display the first OHT and when ready for the answers you overlay
the second. If they are lined up correctly, as described above, the answers will sit clearly in the gaps.
2. The reveal
This technique involves covering part of the OHP with a plain sheet of paper. Because paper is not transparent, any text on
an OHT under the paper cannot be seen. Instead of using an overlay to show the answers to questions, you can have all
the information on one OHT. Line up the piece of paper so it is covering everything except the first question and answer.
When you are ready to go on, move the piece of paper down to reveal the next question. Using this technique means that
the students aren’t looking at all the questions at the same time. You can focus only on the question you want to.
The OHP is a very useful classroom aid but there are a number of practical considerations to think about when using one.
• Too much information!
Although the OHP is a very clear and convenient way of displaying information, try not to use it as the basis of a
classroom lecture. If everything is shown on your OHT the students might get bored because all the information is there
and they won’t need to listen to you. So, either use it with the blackout technique and just reveal each point as you’ve
talked about it, or put it up at the end of your lesson as a reminder and a summary.
• Placement and focussing
For the projected image to be clear and easy to read, the OHP has to positioned carefully and focussed manually. It’s best
to set this up, if possible, before the start of the class.
• Bulb
Many OHP’s have a spare bulb inside the body of the projector. It is not uncommon for OHP bulbs to ‘blow’. If this happens
in the class all is not lost if you know how to find and change the old bulb for the replacement one.
• Pens
There are many different coloured pens available for use on OHTs. They come in different thicknesses. Be careful though as
some pens are ‘permanent’ - this means that once you write on an OHT you cannot erase what you have written. If you
want to be able to clean and reuse OHTs be sure to use a temporary marker.
• Copying on to OHTs
It is possible to obtain OHTs which you can use in a photocopier. However, normal OHTs will cause great damage to a
photocopier. If you want to photocopy on to an OHT, make sure you have the right kind!
• Light
As the image from an OHP is projected light, it is often necessary to darken the room so that the information can be seen
clearly. When using an OHP be prepared to turn off some or all of the lights and close curtains and blinds.
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