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5.48 A degree of indefiniteness is permissible in claims. Indeed, a purposive construction according to the principles discussed in Catnic allows for such imprecision in some cases.
5.49 The appropriateness of imprecise terms such as “substantially”, “about”, “more or less” and “approximately” will depend on the specific circumstances of the case. For example, the definition of a temperature of “about 50 degrees” may be appropriate since in practical terms the person skilled in the art would not expect that precise temperatures could be achieved under standard operating conditions. Where terms such as “about” are used, the degree of variance from the defined value will depend on what the person skilled in the particular art would understand it to mean. In the case of a temperature of about 50 degrees, this might mean 51 or 52 degrees. However, in a definition of about 20K there may be a more significant variance. Conversely, in a feature defined to several decimal points, the degree of variance may be more restricted.
5.50 In some cases the use of indefinite terms is objectionable. For example, a definition of a radical having “about 6 carbons” in a chemical compound would be unclear since in the chemical field a more precise definition might be expected. This may be a different consideration in the area of polymers where a product may comprise a mixture of polymers of various lengths.
5.51 In general, an objection should only be raised if the use of an indefinite term introduces an ambiguity in the scope of the claim (that is, the skilled person would be unable to reasonably determine the scope of the claim), or if the invention is not clearly distinguished from the prior art with respect to novelty and inventive step (such as where there is only a relatively small difference between the range defined in a claim and a disclosure in the prior art – usually in combination with a consideration of inventive step).
5.52 Generalising expressions such as “substantially” may be allowable if it does not render the scope of the claims indeterminate. If the word “substantially” merely indicates that the patentee is not limiting his monopoly to that precisely shown in the drawings and description, then the term may be allowable.