Segment Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 28, 2013	Jun. 29, 2012	Jun. 28, 2013	Jun. 29, 2012
Segment Reporting Information [Line Items]
Revenues	$ 75,505	$ 74,484	$ 148,165	$ 146,409
Operating income	17,593	17,361	28,444	29,177
Capital expenditures	1,742	648	3,701	1,942
Depreciation and amortization	1,235	1,182	2,384	2,326
Engineering and other scientific
Segment Reporting Information [Line Items]
Revenues	53,635	53,996	106,958	105,713
Operating income	16,758	16,423	33,632	31,356
Capital expenditures	1,573	458	2,934	936
Depreciation and amortization	757	700	1,456	1,384
Environmental and health
Segment Reporting Information [Line Items]
Revenues	21,870	20,488	41,207	40,696
Operating income	7,900	7,005	13,480	13,789
Capital expenditures	31	31	69	174
Depreciation and amortization	71	69	145	137
Operating segments
Segment Reporting Information [Line Items]
Operating income	24,658	23,428	47,112	45,145
Capital expenditures	1,604	489	3,003	1,110
Depreciation and amortization	828	769	1,601	1,521
Corporate
Segment Reporting Information [Line Items]
Operating income	(7,065)	(6,067)	(18,668)	(15,968)
Capital expenditures	138	159	698	832
Depreciation and amortization	$ 407	$ 413	$ 783	$ 805